Exhibit 2.1


                      AMENDMENT TO STOCK PURCHASE AGREEMENT

                  THIS AMENDMENT AGREEMENT dated as of November 28, 1998 is made
and entered into by and between APOGEE ENTERPRISES, INC., a Minnesota
corporation ("Seller"), COMPUDYNE CORPORATION, a Nevada corporation
("Purchaser") and HARMON, LTD. (formerly known as W.S.A., Inc. and referred to
hereinafter as "WSA"), a Minnesota corporation.

                  WHEREAS, Seller and Purchaser are parties to a certain Stock
Purchase Agreement dated as of November 10, 1998 (the Stock Purchase
Agreement"); and

                  WHEREAS, Seller and Purchaser desire to amend the Stock
Purchase Agreement and to include WSA as a party thereto for a single limited
purpose; and

                  WHEREAS, WSA desires to become a party to the Stock Purchase
Agreement, as amended, for a single limited purpose.

                                    AGREEMENT

                  NOW, THEREFORE, in consideration of the mutual covenants set
forth in this Amendment Agreement, and for other good and valuable
consideration, the receipt and sufficiency of which are hereby acknowledged, the
parties hereto agree as follows:

1. The first sentence of the initial paragraph of the Stock Purchase Agreement,
naming the parties to the Stock Purchase Agreement, is hereby amended to read as
follows:

                                    THIS AGREEMENT dated as of November 10, 1998
                  is made and entered into by and among APOGEE ENTERPRISES,
                  INC., A Minnesota corporation ("Seller"), COMPUDYNE, INC., A
                  Nevada corporation ("Purchaser"), and, solely for the purposes
                  of Section 1.01 hereof, HARMON, LTD. (formerly known as
                  W.S.A., INC. and referred to hereinafter as "WSA").

2. The first sentence of the introductory portion of Section 1.01 of the Stock
Purchase Agreement is hereby amended by adding to the end thereof, the
following:

                  "and, in connection therewith, at Closing WSA hereby transfers
                  to the Purchaser all of whatever right title and interest it
                  may have in and to the benefits and Liabilities in the
                  Transferred Projects (defined in subsection (a) below)."


3. The second sentence of the introductory portion of Section 1.01 of the Stock
Purchase Agreement is hereby amended by deleting the first clause thereof and
inserting in lieu thereof the clause "As a result of such purchase,".

 
4. The introductory language of Section 1.03 of the Stock Purchase Agreement is
hereby amended to read as follows:

                  "After the Closing, the Purchaser will (with respect to
                  subsection (c) below) and will cause the Companies to be
                  responsible and obligated for and will (with respect to
                  subsection (c) below) and will cause the Companies to timely
                  pay and discharge the following Liabilities (the "Company
                  Liabilities") relating to the Business:"

5. Section 5.04 of the Stock Purchase Agreement is hereby amended to read as
follows:

                   "Purchaser shall use reasonable commercial efforts to provide
                  bonding for the Purchaser's payment and performance
                  obligations under Section 1.03(c) hereof in substantially the
                  form set forth in Schedule 5.04 hereto (the "Bonding
                  Condition") with respect to the Transferred Projects
                  identified on Exhibit A attached to Schedule 5.04."

6. The definition of "Liabilities" in Section 10.01 of the Stock Purchase
Agreement is hereby amended to insert the parenthetical "(including, without
limitation, the obligations of payment and performance)" after the word
"obligations" therein.

7. Schedule 5.04 of the Stock Purchase Agreement is hereby amended by deleting
such schedule and substituting in lieu thereof the new Schedule 5.04 attached
hereto as Attachment A.

8. The Stock Purchase Agreement is hereby amended by adding thereto a new
Section 11.17, as follows:

                  "11.17. Additional Notice Requirements: If Seller receives
                  notice, in any form, of any claim or potential claim with
                  respect to any of the Transferred Projects, or any dispute
                  relating to any contract with respect to any of the
                  Transferred Projects, then Seller shall promptly forward
                  written notice of same to the Purchaser, and, if any such
                  notice relates to a Transferred Project identified on Schedule
                  5.04, Seller shall also notify Liberty Bond Services sent by
                  hand delivery, or by registered or certified mail, return
                  receipt requested, first class postage prepaid, or by a
                  nationally recognized overnight delivery service, or by
                  telecopy (followed by a hard copy sent by one of the foregoing
                  methods), addressed to Liberty Bond Services, Surety Law
                  Department, 600 West Germantown Pike, Suite 300, Plymouth
                  Meeting, Pennsylvania 19462."

 
                  IN WITNESS WHEREOF, this Agreement has been duly executed and
delivered by the duly authorized officer of each party hereto as of the date
first above written.

                                      COMPUDYNE CORPORATION


                                      By:_________________________________
                                         Martin A. Roenigk
                                         Chairman and Chief Executive Officer

                                      APOGEE ENTERPRISES, INC.



                                      By:_________________________________
                                         Robert G. Barbieri
                                         Vice President Finance and
                                         Chief Financial Officer

                                      HARMON, LTD.



                                      By:__________________________________
                                         Michael A. Bevilacqua
                                         Treasurer