Did you find any red oxide visible?
My car was built a few months after yours, same plant. I have found no visible red oxide. I feel I'm going to be ex-communictaed from the church of concourse mustang if I finish my restoration with none visible.
FWIW, what I could uncover about mine, it looked just like yours. Near as I can tell, which is hard with all the white over it, the bottom was actually black. I guess it could be slop, whichever it is, once the white overspray is on it, I'm not sure if it will matter much.
Here was my first shot at reproducing what I saw on mine. I'm redoing as some of this white was just too dry.