Ran across an obscure little tidbit I hadn't noticed before in the '68 shop manual. In the carb specs section it lists choke cover markings for Holley 4150-C carbs. The applications are as follows:
C8OF-C (390GT M/T) - "GT1"
C8OF-D (390GT A/T) - "GTA"
C8AF-AD (427 A/T) - "57M1"
No info on 428CJ covers - my manual was printed in Nov. 67.
I'm posting this because I could only find one indirect reference in another thread. Maybe this can serve as a one-stop reference for folks seeking this info in the future.
I checked my recently acquired C8OF-D carb, and it has the correct "GTA" cap. I have another one in storage that I haven't checked yet. I don't have any photos handy of my own covers, but I did find a "GTA" in an auction that I'm including below. Sorry, couldn't find pics of the other markings.