Dearborn Primer Color 1968 Mustang

I am doing a rotisserie restoration on my 1968 Mustang Fastback that was built at the Dearborn factory the third week of June, 1968. My car is about to have the bottom primeed, painted and then primed again, however I have not been able to determine which color the primer should be. When I stripped the car I found absolutely no traces of red oxide primer, other than paint, the only other color I found appears to be black.
I have been told that San Jose used red oxide, and that Dearborn and Metuchen used a gray/black primer is this correct?
Does anyone know where I can get an accurate Dearborn color primer?

Thanks,
Darren