The website OldMouse.com used to have a page talking about it, but for some strange reason, they have removed it! Only a few days ago, Google still had cache of the page but they have since also removed it.
Thankfully, Archive.org still has it, at least as of the time of this post.
This is what the page said:
Xerox-Microsoft Hybrid Mouse
This mouse inspires speculation about the feedback Xerox gave Microsoft for the mouse it commissioned.
This Xerox mouse for sale on eBay by user vintagecomputermuseum appears at first glance as Microsoft's so-called grey eyed mouse simply rebranded for Xerox in custom coffee colors. The Microsoft grey-eyed mouse was released in 1985. Apparently Xerox commissioned this custom mouse after abandoning its own Xerox 8010 (so-called Star) optical mouse of 1981 origin. The custom connector plug that looks like an RJ45 would not work on a regular PC.
But flip the Xerox mouse over and its ball and housing retain the style of Microsoft's earlier so-called green eyed mouse (developed in 1982 and introduced in 1983). This Xerox mouse wears the 1985 Microsoft mouse's shell on the 1983 Microsoft mouse's mechanism.
The grey-eyed Microsoft Mouse 5.0 of 1985 release has a rubber coated steel ball held in place by a round plastic retainer ring that latches into a round track. The older mouse has a bare steel ball held in place by a screw tab on its retainer ring, just like the ball housing on this Xerox branded mouse.
