I don't think it is olive drab because that is very green although flat and darker. The parts like the retainers and 69/70 cowl wiper arms are more bronze then green. I never gave the zinc dicromate a second thought until I saw some examples of the dark plated . Since the retainer was part of a electrcal assembly like many other light retainers that are zinc dichromate which is typical to electrical I never thought another plating was used.
I believe the dark bronze finish might be very close to the 65-66 seat riser plates that we just had tested. At least that might be a good place to start
Again, as others have stated, might just be a batch or to of plating solution that were mixed differently