This was a "Mouse Systems" design. Early mechanical mice commonly had a switch labeled PC/MS which would toggle the protocol used by the device. https://en.m.

This was a "Mouse Systems" design. Early mechanical mice commonly had a switch labeled PC/MS which would toggle the protocol used by the device. https://en.m.wikipedia.org/wiki/Mouse_Systems https://en.m.wikipedia.org/wiki/Mouse_Systems I own one somewhere in a closet.