Per randyjawa's query, do post the serial number. Peugeots of that era had a small plate riveted to the bottom of the BB shell. That was either removed (two telltale rivet holes will remain, unless filled) or painted over. A factory (not owner-applied) serial number stamped into the BB shell or elsewhere on the frame would indicate a different pedigree.

The fixed cup threading may tell us something, as well, because Peugeot was the last major French manufacturer to switch from French to Swiss threading (1979-1980). Motobecane got it right (left?) a few years earlier and was an early adopter of SunTour derailleurs. As others suggested, if the derailleur hanger was indeed modified from Simplex to SunTour, that would be another possible hint.