according to my '65 TIM&JG, it was suppose to be:
fiberglass, primer, body color, undercoating, than black-out with semi-gloss black paint.
I'd guess but maybe someone can confirm that the primer and body color wasn't too heavily applied - maybe mostly overspray from painting the rest of the body?
than undercoating was applied to to reduce damage from rocks and road debris, than the black-out semi-gloss paint was applies to give it a finished look.
on the front wheelwells (such as you are showing it looks to me) was undercoated at the top of the fender panel and about 3" below the fender above the the rubber flap on the side. The undercoating covers the whole back area to the bottom. On the front portion, the undercoating comes down about 6-8" from the top.
this all comes from the '65 TIM&JG.
You can see some of the undercoating still on your wheelwell area off to the right of where you were cleaning and rubbed thru to the 'glass so either the area you are questioning about either never got any undercoating or it had been removed already by previous work on the car.
i'd just clean it as good as possible and hit the underside with black semi-gloss paint to give it a nice freshened up look.
