
there,
Sometimes I can be really DENSE!!
I am having trouble visualizing what you are describing,
"kind of hard like a clay" type substance over the top of one rear wheel well...
I've done 3 carpets, one in my own 81 and 2 in other friends 1981's and I do not remember anything under the original carpet except the 1/2" or so insulation/sound elimination material

directly under this material was the fiberglass floor board or inner fender.
Look up under the wheel well of the one that has the clay like substance on the inside, you might find that was a repair of some kind
Just when I think I know about everything there is to know about our 1981's
I learn something new
I used the aluminum two sided bubble insulation I bought at Lowes did the entire floor before installing the new carpets
Good luck and from experience, I KNOW your 1981 just love's all this attention and quality time you two are spending together :dance
Bud