Mouse Systems A/3 Mouse. Model No. 403243-00. One of the first optical mice! This version was made for the Mac. As with many early computer items, very little information is available on the internet. I did find this interesting article at Low End Mac about optical ADB mice. This mouse requires the gridded mousepad from Mouse Systems to work. If I run across one, I’ll be sure to try it.
Untested and sold as-is.
Photos are illustrative only. I just turned up a few more and most are very discolored on top.