I can only give my experience with my 1950 CJ. I believe my Jeep had all of it's original parts, wheels, body panels, electrics...etc. I was the first one to "rebuild" it.
The body tub, windshield and fenders had the faintest traces of a thin wash of red oxide primer.
The hood however had a heavy coat of red oxide primer on every surface, and I saw no sign that it had ever had work done to it (no fillers, no hammer work, etc) so I think that was original primer too.
I would not be surprised at the different thicknesses of primer since I think the hood could have come from a different part of the factory floor, different workers, or even a different supplier.
I know GTO's in the 60's had the front fenders and hood primed black, and the "body tubs" were primed with red oxide. They were finish painted that way!
So, that's one data point for you.
Incidentally, my Jeep was Luzon Red.