OK, but the SX660 is not the catalog spec derailleur for the 1982 PH12. The spec derailleur is an SX610 and, if I recall correctly, there was lots of Delrin and steel in the parallelogram arms on that model. The SX660 would be a substantial upgrade over the SX610, if Peugeot made a substitution. It would be interesting to know if the OP has the catalog spec SX610 or an SX660.

BTW, Simplex missed out on a real marketing opportunity by not naming the SX660 an SLJ6500 or something similar to denote that it was an SLJ series derailleur. That's unfortunate because they were great performing derailleurs, especially when paired with the retro-friction levers.