Originally Posted by
clasher
The rear derailleur hangar might be a simplex style which won't work with anything but a simplex derailleur. It's considered gauche by some to modify the frame. I wouldn't do it to a px-10 myself, but would try and use a claw-type adapter on top of the simplex one.
I'm not usually one to suggest modifying a vintage frame, but the older Simplex hangers were a 9mm unthreaded hole, easily tapped to M10 for a standard threaded mounting bolt. This modification does not remove any functionality from the frame, as a Simplex derailleur with an unthreaded mount will work perfectly well in the threaded hole.
I'm not sure a claw over the Simplex hanger would work well, and it would be butt-ugly besides..