Just thought I would add a photo of my original Buck tag and what Marti said the letter information stood for.
1969 Mach1, 2/24/69, Metuchen built. Yes 46 years ago today the Mach was built
I'm assuming the "71" is some sort of rotation number, he didn't say. I know it's not the DSO cause my Mach1 was delivered to the Charlotte DSO (22).
GT=GT suspension, TT=Tu-Tone hood, CL=courtesy lights in doors, RM=remote mirror, MO=rocker molding, HP=hood pins
and 3S=internal plant code.