Both sides would have been painted with a red oxide primer. During the painting process exterior paint would have been applied to the back side of the fender either by overspray or direct application.
Example as the painter was assuring full coverage at the back edge of the fender (so no primer was visible when the door was open and viewed from the door jamb area of the interior) the face were the headlight bucket (and front valances) would attach or around the wheel opening lip paint would have flowed directly on the surfaces behind these surfaces producing a nice finish. Other surfaces would have received a little or allot (really nice paint jobs) depending on the painter. Very few areas (other than hidden behind reinforcements for example) would have received no paint or overspray in my experiences.
The problem you'll find today is that most painters will suggest that they can tape off the back of the fender to control the overspray, and apply it later. It almost never is done correctly this way and often the patterns make no sense as you view the final product. A second issue is that today's spray guns use less air and pressure which produces less overspray and in turn additional challenges for the restorer
Finally if you use base coat clear coat the overspray is difficult to get to look glossy where it should and flat where it should since the single application (as it was done originally) is not being practiced.
Suggest if you're looking at for factory look use single stage paints, don't tape off areas that should get overspray and if you have to tape off the exterior panels and repeat the edge painting until you reproduce the factory look. - Also as you do this keep the gun as close as possible to the same angle since every time you go back over an area the natural paint edges get softer and more blurred
Hope this helps
