http://www.microsoft.com/en-us/news/press/1999/apr99/eyepr.a...
DEC used three button puck-style mice, the one you saw was likely attached to a DECStation and I think DEC had been using them for quite a while, maybe from the mid to late 80s. Here's logitech's first mouse from 1982 which is also a puck.
http://www.oldmouse.com/mouse/logitech/p4.shtml
http://www.microsoft.com/en-us/news/press/1999/apr99/eyepr.a...
DEC used three button puck-style mice, the one you saw was likely attached to a DECStation and I think DEC had been using them for quite a while, maybe from the mid to late 80s. Here's logitech's first mouse from 1982 which is also a puck.
http://www.oldmouse.com/mouse/logitech/p4.shtml