http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ih=017&sspagename=STRK%3AMEWN%3AIT&viewitem=&item=270140607282&rd=1&rd=1

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Nice. I note that the original owner

couldn't find the original 3.5" software diskette. (This mouse) will still function great as a single-button mouse.

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You may well be able to make use of something last seen online as:

three-button-power-251.hqx

The Read Me First! file associated with the control panel found in the above item states that it's the

Macintosh A3 Input Devices3-Button Power™ Control Panel

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.
I have a copy locally, and am willing to send it to you if you can't find it elsewhere.