I think I might have solved the "cut-pile" or "80/20 loop" carpet mystery

cosgig said
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!
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One thing to bear in mind is that we are talking about cars that were built 50 years or more ago. Lotsa carpets got replaced in that time. Sometimes more than once.