The first commercial clear mouse is Logitech's ClearCase ball mouse designed in 1987 and released in 1988, a special edition as part of its popular slim, rectangular C7 series, celebrating shipment of their 2 millionth mouse. Its clear body and three transparent green buttons reveal all the insides, including LED lights.
Logitech billed it as "A mouse with nothing to hide," complete with a large poster labeling all the visible parts.
This mouse seems to be a slightly different model than in the Logitech poster, but here's the idea with a diagram modeled after theirs. [+]
"With standard mice, you only get to feel their power, but this mouse shows it all to you. And, as you watch the mouse work under your hand and on the screen, it'll become apparent to you why Logitech is the world's leading mouse company."
This mouse has a DE-9 socket Lavelle (500059) plug with deep ridges on the finger screws.
ClearCase Mouse S/N 27976 FCC ID DZL6QBC MADE IN TAIWAN OCTOBER 1, 1988
A plastic impression inside the case says LOGITECH 1987.