As was previously mentioned, these use a braze-on that is
essentially a single bottle cage braze-on. They definitely
used them in the '83-'85 period, including on some relatively
decent bikes, e.g. the PSV-10 (which was built from Super
Vitus tubing) and the PGN-10 (from Reynolds 501). Apparently
they began using them a bit earlier and on bikes of lower
quality levels as well.
Replacing them with another deraileeur can sometimes be a problem as the braze-on fitting might interfere with the band a clamp-on derailleur. It all depends on the chainring size and the partiular derailleur in question. On my PGN-10 I had to use one of the older Ultegra models that use the top swing design.
Anyway, Peugeot was using these at the same time that they (and Motobecane) experimented with alternative styles of clamps for the seat post. Those are also great fun to replace...
Fred Rednor - Arlington, Virginia (where it was nice
to ride today despite the wind, as long as you wore