There were many different (paint) coatings, and many different gloss levels of black. Items like the subframe and control arms were painted by their suppliers, and parts like bumper brackets, engine accessory drive brackets, rear shock plates, and things of that nature were received raw and were dip-primed or flow-coated with black primer after cleaning and phosphating at the assembly plants. Front sheet metal (hoods, fenders, headers, valances, inner fenders, radiator supports, grille supports, etc.) were also received raw, and were dip-primed or flow-coated with black primer after cleaning and phosphating.
There's no "EPDM" coating - what you're referring to is "E-coating", which was the successor process to dip-priming and flow-coating; E-coating is also a dip-prime process, but uses positive and negative electrical charges in the paint and on the parts to get a more uniform coating.
Service sheet metal in the 60's was shipped raw in racks from the stamping plants to the Regional Parts Depots; each depot had its own dip-prime system for painting those parts before boxing them for storage, and that same method is used today, except the old dip-prime systems have been replaced with E-coat systems.