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I suspect it is raw fibreglass with undercoating in places. I just cleaned the wheelwells on my 73 and that's what I found. I spoke with a vette bodyman last night who tells me the undercoating was there to protect the glass from stone damage. Apparently, a stone hitting the underside of a fender without u/coating could cause the topside paint to "spider".
I plan to paint the underbody of my car including the w/wells a slate gray color to match the glass look and re-apply the U/coating as close to the factory look as possible.
Wheels got a spritz of semi-flat black during blackout, but application was uneven and spotty at best. The entire well will not be evenly painted. Fronts and rears will show the most paint; sides and top little to none.
Not sure '68s got any type of undercoating; later years did.
If you're not going the NCRS scoring route, just use a hi-pressure soap spray and flush on the wells, let them dry and spray 'em with Krylon semi-flat black. That's probably not "by the book", but it sure makes them look like new. It is surprising how much better the whole car looks after the wells are finished. I guess the brain just expects to see crud in there.