Rear derailleur appears identical to the SX410 on my 1979 PKN10 which also had a "07 79" date stamped on the spindle of Bottom Bracket, so check there for a date when doing the overhaul. Brochures show 77 PX10 had a Stronglight 99 crankset, PRN10 had the Stronglight 49D.
Peugeot Brochures - USA
Don