I think I might have solved the "cut-pile" or "80/20 loop" carpet mystery
This 1973 Dart Sport was “sporting” the original carpet when I got it. It was cut pile carpet, so I assume by 1973, cut pile was the choice.
However, when I got this 1973 Roadrunner, it had level loop.
This 1979 Roadrunner had cut pile.
No rhyme or reason it seems!