Mark Bulgier wrote, as an aside on his free PX-10, that it has <snip> "Simplex dropouts not threaded for derailleur (so Simplex rears only unless you want to modify it)."
Some one long ago showed me that careful use of a plumber's tapered tap (1/4" ??) works well enough for this application (although I since bought a proper 10 mm tap). Pitch is close enough, given that only about 2 - 3 threads are engaged, but you have to be careful not to enlarge the hole too much. A Dremel or similar tool works to hack the dropout and leave the lip for the Campy/Shimano/everyone else stop.
Obviously, one doensn't do this kind of brutal aggression on a Singer, Herse, or other work of beauty, but my conscience is clean wrt PX-10s in poor condition.
harvey "scrounge" sachs
mcLean va