The 1978 DEC10 Prolog user guide mentions PROG.PL as an example of a Prolog file with an extension. See 3.2 in https://userweb.fct.unl.pt/~lmp/publications/online-papers/U...
Maybe the 1975 Prolog I manual has more to say...
Maybe the 1975 Prolog I manual has more to say...
I think it's even more explicit on p25 where it says "A module name can have the form either file.ext or just file. In the latter case the extension 'PL' is implied."