The other thread is useful.
From memory, AP (Lockheed, Borg and Beck) stopped using cadmium plating in the late '50s/early '60s.
The plating they did in the factory was a passivated zinc. -This was a treatment to the zinc to improve the corrosion resistance.
The length of time the part was processed improved the protection: silver was the lowest level, the gold, olive, and black.
Obviously the silver (or 'bright') was the cheapest.
At various times you would see silver or gold (and sometime both at the same time...)
I don't recall seeing any olive or black plated parts for automotive use...
(My father was one of the foremen in the plating shop. -So I was the only apprentice to ever get the tool box we made gold zinc plated