You sure assembly line upper control arms got P/N stamped on them? Usually over the counter NOS parts get inked stamped. Where as the assembly line does not. Or sometimes one part in a crate of parts gets a stamp or tag to identify the lot??

Do you have original untouched cars to show examples of UCA P/N stampings??

My 68 Barracuda has a white P/N stamp on inside of the drivers fender, but that is because it was replaced with a dealer part.

Glen Ray Radiators said there were at least a two different radicator vendors. At least one vendor had two different plants. I remember one was not in Canada.

Maybe Glen Ray Radiators can explain how to find the radiator vendor code. He is a Moparts member.