First want to mention that the comments above are not directed at NJ built cars. Not allot of unrusted cars are available and what we do have is mixed changing from year to year so at this point its difficult IMHO to make good sense out of the information we have at this point for every year there and those are best dealt with individually
...............Looking at your pictures it does look like the amount of body color in the rear wheel wells did vary quite a bit.
Guess I'm not seeing what your seeing. Please understand that some of the examples have seen up to and over 100,000 miles so wear and tear of the original paint would be considerable, especially long the center of the wheelwell where dirt, gravel and such would be thrown by the tire. Lowest mileage car of the examples would be the second white car picture. Can't see where much of the wheel well would be missed as a painter applied paint to the rear fender lip with a high pressure high volume gun (very different from today's guns) as well as passing the gun over the opening as they painted the quarter panel
Readdressing Chris's comment I thing it will be difficult to prove that any black out of the rear wheelwells was factory. Not sure how you can determine between what earlier owners put on the car and what would have been done at the plant though you can often show when the black out was added by owners like so many of us did. Ford example I've done the pinch welds and rear wheel black on just about every daily drivers I've owned - last one a 2000 Ford truck - body colored pinch welds just stand out too much for me
Just to confirm the order of things in that area: batch primer, then sound deadener, and finally body color- with some batch primer and sound deadener possibly exposed in areas?
If were are discussing just the 70 and just the rear wheel wells, you have batch color to maybe mid way (with some addition overspray outward) out from the frame and then some gray primer overspray from the exterior application (do have a few scrubbed examples that suggest red oxide before these first coats - possibly panels painted prior to their arrival at the plant). Then sound deadener then body color.
You can often find bare steel and or a thin line/edge of sound deadener hidden behind the fender lip where the angle of the exterior color did not allow for application as well as pockets in the floor (between the wheel well and frame rail) where shadows were produced by the wheel well lip, rocker and such
Do you have any Dearborn examples of around May 70?
Looked at what I have but most are dirty with signs of body color across the surface under the dirt- really not as clear as in person, sorry not allot of people have scrubbed their original rear wheel wells and provided them.