Wonder why you never see grease pencil markings on the firewall of Norwood OH produced 1977 Firebirds? I carefully studied our very low-mile unrestored 1977 Trans Am survivor and have found that grease markings were written on the firewall. It is just they were applied on the red oxide primer then sprayed over by the thinly covered black paint used on the firewall. In the proper light I can faintly see 75 which is the color code for our car and is located on the upper passenger side of the firewall.
So those of you who are correctly restoring a 1977 Firebird, yes there were paint codes written on the firewall. They were, however covered over by a thin coating of black paint used on the firewall.
Not sure on 1978 and 1979 Norwood OH built cars but I would say the process would have been the same. I am not sure about Van Nuys built cars at this time.