IIRC, they started off with horizontal headlights, then went to verticle, then back to horizontal, hold on while I get a pic...

Aha! Here it is. 72 GTO:


I dont know that you can really classify Muscle Cars under 'generations' like you can today, where the average one lasts 7 years, although back then, some cars changed practically every year, GTO was one, Camaro, Charger too IIRC.

Ooooo! Heres a cool pic!!