Yep, every car at MT had it's license # written in some kind of a white crayon on the windows somewhere. I believe they also used yellow.

On new cars just the unit # was written on the windows. I found my dads 02 Camaro in the line up this way, #1685 was written in the upper left side of the windshield when it was new.
I have another closer photo that shows the window # better
http://i1171.photobucket.com/albums/...camaroatmt.jpg