Help! ID this Xerox mouse

I have recently come into posession of what certainly appears to be a Xerox mouse and I am wondering if you can help be identify it, that is, what machine it was used on. It has no markings on the case, cable or connector. It has the same appearance as one used on the Alto with three grey buttons, beige case, exactly like the the one pictured here. http://www.newmedianews.com/pictures/1998/10/09/th_alto/3.jpg However, it is an optical mouse, unlike the "rolling ball bearing" mouse described in the Byte magazine article in Sept 1981 here. http://www.byte.com/art/9609/sec4/art3.htm It has a black cable, not too long, about 28", terminated in a grey db-9, not molded, but hand assembled. Here are two pictures of it, taken with my scanner. http://www.his.com/~jlewczyk/MyXeroxMouse.jpg (123K) http://www.his.com/~jlewczyk/MyXeroxMouseBottom.jpg (60K) Thanks in advance for any help you can give me. I'd love to verify if it works! Are pin-outs available for it? John jlewczyk(a)his.com