The upper control arm was positioned in a fixture; the bushings, cross bar, washers and bolts were assembled to it and torqued. The fixture established the position of the bar relative to the arm for assembly to the frame in production. The assembly was then dipped in a black coating. The ball joint was installed afterwards and was not coated. The coating was not intended to last 40+ years; none of the parts had any prep. Don't draw conclusions based on how a part looks today; I have seen plenty of rusty Camaro subframes in my days that were known to have been coated. 44 years is a long time.
Painted upper control arm assemblies are visible in engine bay photos in vintage road tests. Washers were coated.