If one does not mind a bit of elementary machining (mostly grinding), it's easy to hang a rear derailleur on a frame with track dropouts (been there ... done that with a Swedish Avanti and a SunTour V). The derailleur claw is mounted backward. If possible and convenient, grind a stop into what is now the rear-facing curve of the claw; if not, just tighten the derailleur mounting bolt, with a shim or two, if necessary. Works like a champ.
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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