66-72
The original Trans-Am had 2 classes: Under 2 liters and Over 2 liters. The Over 2 liter championships were won by the VTA cars we currently run, with the big exception of the AMC Javelin, which won the championships in 71 and 72 and was 2nd in 70; plus the Pontiac Firebird. A few Cougars ran; the odd Dodge Dart and Pontiac Tempest showed up; and there were less than a handful of showings by a Nova and a Falcon.
The Under 2 liter class was dominated by Porsche, then Alfa Romeo and finally Datsun, when the displacement was increased to 2.5 liters. Tons of different cars ran in under 2 liter -- VWs, NSUs, Cortinas, Austin Coopers, BMW 2002s and on and on.
So the most significant cars that aren't in the HPI catalog at the moment are the Firebird Trans-Am and the Javelin. In historical importance, the Javelin probably wins out.