I have a 1962 Flying Scot touring bike that was mostly original when I received it, the only exception being a Shimano Deore rear derailleur that the original owner had installed. I also could not figure out what derailleur would be appropriate for the time period and the 13-28 freewheel and 40-50-60 (honest) chainrings. Luckily, the former owner recently found the original correspondence regarding the purchase of the bike. He had requested that the bike be fitted with a Cyclo rear derailleur, but the builder could not make that work with the wide range gearing and substituted a Huret Allvit. I never would have guessed that an Allvit would be the choice for this bike, but that's what it came with. I have recently purchased one and look forward to trying it myself.
Mike Fennelly
Alameda, CA