The K75S has a square headlight.
(http://i1221.photobucket.com/albums/dd470/Motobrick/gen_maint/k75smech.jpg)
The K75C has a round headlight.
(http://images.psndealer.com/dealersite/images/bmwmotorcyclesdallas/VP1457233_1.jpg)
K75RTs have larger K100 gas tanks and radiators than the other K75 models.