Just checked a box of bolts & misc. from a '69 Torino GT I parted out in the mid 80's. The trunk latch is marked:
mmp
ROCKFORD ILL
The non-rusted areas have a shiny plating, appears to be zinc (not shiny enough to be cadmium, in my opinion). The latch is heavily worn, its condition matches the other parts I have, and this car was NOT a vehicle anyone would have put a repro part in! It was heavily rotted to the point the entire front end was flexing at the firewall - not safe to drive. So I have a high degree of confidence this is the original latch.
I don't have the VIN number written anywhere, so I don't know the assembly plant, in case plants obtained latches from different suppliers.
FYI the "patchwork" finish you describe sounds like hot-dip galvanized.