there were 2 ford part #s for the 2 versions of hinges c7zz first then c9zz & that had nothing to do with the rivet change in fall 69.
Not exactly true. The factory (per the assembly manuals) used C7ZB-16800,1 for 1967 and 1968 Mustangs, C9ZB-16800,1 for 1969 and 1970. Any part number with a "Z" in the fourth position is a service part number and will cross-reference to what Ford considered as a replacement part, not necessarily what the factory used. In Ford Car Parts, C9ZZ-16796,7 was the replacement hinge for 1967 thru 70. Unfortunately there are "part number" problems:
1 P/N C7ZB-16800,1 and C9ZB-16800,1 are assemblies, hinges and springs, the actual hinge part numbers are "invisible". A running change or a different part number for the successive year are not known.
2 P/N C9ZZ-16796,7 as a service part without the springs, is shown to fit all 67-70 Mustangs.
The bottom line is simple, pictures showing the specific differences with year and build date (plant also?) noted. I don't think it does anyone a service to simply say "it's different" without some back-up.
Jim