Earliest known..??.

[JoeC]

The Jenkins cars changed a lot race to race if you look at the pictures. Look for emblems, parking lights, side lights (68) , grills - sometimes they were there , sometimes not there or modified. Fiberglass fenders and front parts were allowed in some classes.
The cars were changed for the rules for NHRA, AHRA, Match race, Modified production , SS/C Super Stock , Pro Stock, Gasser etc.

Here is a photo of the 67 with the RS grill , no fender emblems, but mostly this car was run with standard grill and 396 fender emblems.