SECURITIES AND EXCHANGE COMMISSION
                              WASHINGTON, DC 20549

                                  SCHEDULE TO/A

                                (AMENDMENT NO. 3)

        TENDER OFFER STATEMENT UNDER SECTION 14(D)(1) OR 13(E)(1) OF THE
                         SECURITIES EXCHANGE ACT OF 1934

                               (FINAL AMENDMENT)
                                      AND
                                  SCHEDULE 13D
                   UNDER THE SECURITIES EXCHANGE ACT OF 1934
                               (AMENDMENT NO. 18)

                         National Property Investors 6
- --------------------------------------------------------------------------------
                        (Name of Subject Company (Issuer))

                   Apartment Investment and Management Company
                                 AIMCO-GP, Inc.
                          NPI Equity Investments, Inc.
                             AIMCO Properties, L.P.
- --------------------------------------------------------------------------------
                      (Names of Filing Persons - Offerors)

                            Limited Partnership Units
- --------------------------------------------------------------------------------
                           (Title of Class Securities)

                                      None
- --------------------------------------------------------------------------------
                       (CUSIP Number of Class Securities)

                                 Martha L. Long
                   Apartment Investment and Management Company
                                55 Beattie Place
                                 P.O. Box 1089
                        Greenville, South Carolina 29602
                                 (864) 239-1000
- --------------------------------------------------------------------------------
           (Name, address, and telephone numbers of person authorized
       to receive notices and communications on behalf of filing persons)

                                    Copy to:

                                 Joseph A. Coco
                    Skadden, Arps, Slate, Meagher & Flom LLP
                                Four Times Square
                            New York, New York 10036
                                 (212) 735-3000

                                       and

                              Jonathan L. Friedman
                    Skadden, Arps, Slate, Meagher & Flom LLP
                             300 South Grand Avenue
                          Los Angeles, California 90071
                                 (213) 687-5000



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                            Calculation of Filing Fee

<Table>
<Caption>
Transaction valuation*                            Amount of filing fee
- ----------------------                            --------------------
                                               
$1,723,795.09                                           $ 218.40
</Table>


*        For purposes of calculating the fee only. This amount assumes the
         purchase of 36,467 units of limited partnership interest of the subject
         partnership for $47.27 per unit. The amount of the filing fee,
         calculated in accordance with Section 14(g)(1)(B)(3) and Rule 0-11(d)
         under the Securities Exchange Act of 1934, as amended, equals $126.70
         per million of the aggregate amount of cash offered by the bidder.

[X]      Check the box if any part of the fee is offset as provided by Rule
         0-11(a)(2) and identify the filing with which the offsetting fee was
         previously paid. Identify the previous filing by registration statement
         number, or the Form or Schedule and the date of its filing.

<Table>
                                            
Amount Previously Paid: $218.40                Filing Party: AIMCO Properties, L.P.

Form or Registration No.: Schedule TO/13E-3    Date Filed:   November 4, 2004
</Table>

[ ] Check the box if the filing relates solely to preliminary communications
made before the commencement of a tender offer.

Check the appropriate boxes below to designate any transactions to which the
statement relates:

[X] third-party tender offer subject to Rule 14d-1

[ ] issuer tender offer subject to Rule 13e-4

[X] going-private transaction subject to Rule 13e-3

[X] amendment to Schedule 13D under Rule 13d-2

Check the following box if the filing is a final amendment reporting the results
of the tender offer: [X]



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CUSIP No.   NONE

1.       NAME OF REPORTING PERSONS:  AIMCO PROPERTIES, L.P.
         S.S. OR I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (entities only):
         84-1275721

2.       CHECK THE APPROPRIATE BOX IF A MEMBER OF GROUP
         (a)     [ ]
         (b)     [X]

3.       SEC USE ONLY

4.       SOURCE OF FUNDS
         WC

5.       (CHECK IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO
         ITEMS 2(d) OR 2(e))                                                 [ ]

6.       CITIZENSHIP OR PLACE OF ORGANIZATION
         Delaware

NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH

         7.       SOLE VOTING POWER
                  --

         8.       SHARED VOTING POWER
                  75,205 Units

         9.       SOLE DISPOSITIVE POWER
                  --

         10.      SHARED DISPOSITIVE POWER
                  75,205 Units

11.      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
         75,205 Units

12.      CHECK IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES
                                                                             [ ]

13.      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
         Approximately 68.6%

14.      TYPE OF REPORTING PERSON
         PN


                                       3



CUSIP No.   NONE

1.       NAME OF REPORTING PERSONS:  AIMCO-GP, INC.
         S.S. OR I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (entities only)

2.       CHECK THE APPROPRIATE BOX IF A MEMBER OF GROUP
         (a)     [ ]
         (b)     [X]

3.       SEC USE ONLY

4.       SOURCE OF FUNDS
         Not Applicable

5.       (CHECK IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO
         ITEMS 2(d) OR 2(e))                                                 [ ]

6.       CITIZENSHIP OR PLACE OF ORGANIZATION
         Delaware

NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH

         7.       SOLE VOTING POWER
                  --

         8.       SHARED VOTING POWER
                  75,205 Units

         9.       SOLE DISPOSITIVE POWER
                  --

         10.      SHARED DISPOSITIVE POWER
                  75,205 Units

11.      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
         75,205 Units

12.      CHECK IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES
                                                                             [ ]

13.      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
         Approximately 68.6%

14.      TYPE OF REPORTING PERSON
         CO


                                       4



CUSIP No.   NONE

1.       NAME OF REPORTING PERSONS:  APARTMENT INVESTMENT AND MANAGEMENT COMPANY
         S.S. OR I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (entities only):
         84-1259577

2.       CHECK THE APPROPRIATE BOX IF A MEMBER OF GROUP
         (a)     [ ]
         (b)     [X]

3.       SEC USE ONLY

4.       SOURCE OF FUNDS
         Not Applicable

5.       (CHECK IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO
         ITEMS 2(d) OR 2(e))                                                 [ ]

6.       CITIZENSHIP OR PLACE OF ORGANIZATION
         Maryland

NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH

         7.       SOLE VOTING POWER
                  --

         8.       SHARED VOTING POWER
                  75,205 Units

         9.       SOLE DISPOSITIVE POWER
                  --

         10.      SHARED DISPOSITIVE POWER
                  75,205 Units

11.      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
         75,205 Units

12.      CHECK IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES
                                                                             [ ]

13.      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
         Approximately 68.6%

14.      TYPE OF REPORTING PERSON
         CO


                                       5



CUSIP No.   NONE

1.       NAME OF REPORTING PERSONS:  AIMCO IPLP, L.P.
         S.S. OR I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (entities only):

2.       CHECK THE APPROPRIATE BOX IF A MEMBER OF GROUP
         (a)     [ ]
         (b)     [X]

3.       SEC USE ONLY

4.       SOURCE OF FUNDS
         Not Applicable

5.       (CHECK IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO
         ITEMS 2(d) OR 2(e))                                                 [ ]

6.       CITIZENSHIP OR PLACE OF ORGANIZATION
         Delaware

NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH

         7.       SOLE VOTING POWER
                  --

         8.       SHARED VOTING POWER
                  48,033 Units

         9.       SOLE DISPOSITIVE POWER
                  --

         10.      SHARED DISPOSITIVE POWER
                  48,033  Units

11.      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
         48,033  Units

12.      CHECK IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES
                                                                             [ ]

13.      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
         Approximately 43.8%

14.      TYPE OF REPORTING PERSON
         PN


                                       6



CUSIP No.   NONE

1.       NAME OF REPORTING PERSONS:  AIMCO/IPT, INC.
         S.S. OR I.R.S. IDENTIFICATION NOS. OF ABOVE PERSONS (entities only):

2.       CHECK THE APPROPRIATE BOX IF A MEMBER OF GROUP
         (a)     [ ]
         (b)     [X]

3.       SEC USE ONLY

4.       SOURCE OF FUNDS
         Not Applicable

5.       (CHECK IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO
         ITEMS 2(d) OR 2(e))                                                 [ ]

6.       CITIZENSHIP OR PLACE OF ORGANIZATION
         Delaware

NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH

         7.       SOLE VOTING POWER
                  --

         8.       SHARED VOTING POWER
                  48,033  Units

         9.       SOLE DISPOSITIVE POWER
                  --

         10.      SHARED DISPOSITIVE POWER
                  48,033  Units

11.      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
         48,033  Units

12.      CHECK IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES
                                                                             [ ]

13.      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)
         Approximately 43.8%

14.      TYPE OF REPORTING PERSON
         CO


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        AMENDMENT NO. 3 TO SCHEDULE TO/AMENDMENT NO. 18 TO SCHEDULE 13D


         This Statement constitutes (a) Amendment No. 3 to the Tender Offer
Statement and Rule 13e-3 Transaction Statement on Schedule TO, as amended by
Amendment No. 1 and Amendment No. 2 thereto (the "Schedule TO"), filed by AIMCO
Properties, L.P., AIMCO-GP, Inc., Apartment Investment and Management Company
and NPI Equity Investments, Inc. relating to the offer by AIMCO Properties, L.P.
to purchase units of limited partnership interest ("Units") of National Property
Investors 6, a California limited partnership (the "Partnership"), at a price of
$47.27 per unit in cash, subject to the conditions set forth in the Litigation
Settlement Offer dated November 4, 2004, and in the related Letter of
Transmittal (which, together with any supplements or amendments, collectively
constitute the "Offer"); and (b) Amendment No. 18 to the Schedule 13D filed by
AIMCO Properties, L.P., AIMCO-GP, Inc., Apartment Investment and Management
Company, AIMCO IPLP, L.P. and AIMCO/IPT, Inc. (the "Schedule 13D"). Copies of
the Litigation Settlement Offer and the Letter of Transmittal are filed as
Exhibits (a)(1) and (a)(2), respectively, to the Schedule TO. The item numbers
and responses thereto below are in accordance with the requirements of Schedule
TO. Unless defined herein, capitalized terms used and not otherwise defined
herein have the respective meanings ascribed to such terms in the Litigation
Settlement Offer.

ITEM 8. INTEREST IN SECURITIES OF THE SUBJECT COMPANY.

         At midnight, New York City time, on December 28, 2004, the Offer
expired pursuant to its terms. A total of 2,072 Units, representing
approximately 1.89% of the outstanding Units, were validly tendered and not
withdrawn pursuant to the Offer. AIMCO Properties, L.P. has accepted for payment
all of those Units.



                                       8



                                    SIGNATURE

           After due inquiry and to the best of its knowledge and belief, the
undersigned hereby certify that the information set forth in this statement is
true, complete and correct.


Date:  January 10, 2005
                                        AIMCO PROPERTIES, L.P.

                                        By:  AIMCO-GP, INC.
                                             Its General Partner


                                        By:   /s/ Martha L. Long
                                              -------------------------------
                                              Martha L. Long
                                              Senior Vice President


                                        AIMCO-GP, INC.


                                        By:   /s/ Martha L. Long
                                              -------------------------------
                                              Martha L. Long
                                              Senior Vice President


                                        APARTMENT INVESTMENT AND MANAGEMENT
                                        COMPANY

                                        By:   /s/ Martha L. Long
                                              -------------------------------
                                              Martha L. Long
                                              Senior Vice President


                                        NPI EQUITY INVESTMENTS, INC.


                                        By:   /s/ Martha L. Long
                                              -------------------------------
                                              Martha L. Long
                                              Senior Vice President


                                        AIMCO IPLP, L.P.

                                        By:  AIMCO/IPT, INC.
                                             Its General Partner


                                        By:   /s/ Martha L. Long
                                              -------------------------------
                                              Martha L. Long
                                              Senior Vice President


                                        AIMCO/IPT, INC.


                                        By:   /s/ Martha L. Long
                                              -------------------------------
                                              Martha L. Long
                                              Senior Vice President





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