It's not mentioned in my Shop Manuals, but by my eye, it's not painted.
It's FRP (fiberglass reinforced polyester) and the colorant was blended into
the resin.  They probably used carbon black.  That was and still is about
the cheapest and most durable way to do it.

There is one very knowledgeable and experienced Mustang scholar and builder
that may be able to provide the answer you seek, but he no longer
participates or lurks here.  There are other places that he can be found,
but you must search him out to prove your worthiness.

Otherwise, I any premium flat black done in a matte finish should be close.