have no proof because I have no 71 models. I have always been told they used red oxide in NJ. for all years of mustang built there by people at Mustang shows. I will do some asking around to see if I can find someone with an early original from NJ.
I do not think I have ever even gone to look at a 71 for sale. I prefer 72 73.
I will get in touch with Scotty Strickland he has the Mustang junk yard maybe he can share some first hand info, I cannot, just what I have been told by others.
Need to go find a copy of the Ford Facts book and see if it tells like in later years what they sprayed. It does not tell color on later years but number of coats.
Went looking for info on primer used in NJ but found nothing for now. There is a copy of the MCA rules which I hate because they are so vague they are on this site. They allow Gray, Red Oxide and black.
The primer was not the Slop paint it was primer. The Slop gray was what came from purging lines to change colors and what color paint that was caught and recycled.
The primer was primer made for that use not the mixed up Slop. When you mix all colors of paint you always get gray of some tone. If you did photo work in the film days you carried a 40% Gray Card. That was the average between white and black and you metered you light setting off of it.
Gray primer is like the best neutral color that covers the easiest with most colors.