I have two original paint cars and both are gray with over spray of body color. Unless you have the Decor Group then the black out on outside.
I think the body was actually sprayed with gray primer not the slop gray. The slop gray was used on bolt on pieces that did not receive any primer just the slop gray.
For those wondering why it is called slop gray. Ford use to recover all the paint and just mix all the colors together and they refered to it as SLOP. You can order from any mustang parts supplier.
Shock tower braces, bracket for hood latch and some grill parts and the under fender shock tower cap were some of the slop gray parts.
Years ago our local parts house, that sold automotive paint, kept all his screw ups and would do the same thing. Mix it all together and sell to farmers and such to paint equipment with. If you mix a bunch of colors together you naturally get a neutral gray or the Slop Gray of Ford.