I believe there have been a few different thread and diameter standards for derailleur hangers. My usual solution is to mount the desired derailleur using whatever hanging bolt from my parts bin threads properly into my frame (e.g. a SunTour derailleur on a Simplex dropout, with a Simplex bolt instead of a SunTour).
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069