There was a number of derailleurs in the 1930s and into the 40s that use th e striking fork mounted on the top of the chainstay. Possibly the idea was inspired by Campag's double-rodded Paris-Roubaix, or whatever it was called - the one that required back-pedalling to shift the chain. Or did that pos tÚte the gears I am referring to. Remember too, the Manufrance had used a system of retropedallage in the 30s..or possibly earlier.
It's pity that this Ebay seller does not show better photos ie of the chain tensioning system; that would have helped to identify this particular mode l.
however...noticing that the seller lives on or near the Cote d'azur, in Pro vence, my best guess is that this derailleur is one called the GIANELLO. As such it will have a tension arm slung undreneath the bracket in the style of the Super Champion Osgear.
It was invented by a well=known Pro of the era, Dante Gianello, and Itali an who in the early 30s moved into France, lived in Nice and took up French citizenship....much in the way that the URAGO family..and Rene VIETTO had done..
Gianello was one of the outstanding mountain climbing cyclists of his ge neration, a facilioty that helped him build up an impressive palmares. He started out in 1934, turned Pro a couple of years later..and managed to keep him self very busy throughout WWII as a Pro, riding for teams such as Peugeot - his first contract - Genial Lucifer,Urago, F. Pelissier, mercier, Helyett, and ending up for four or five years in the formidable France-Spo rt team. Presumably he rode alongside Vietto, the Lazarides brothers.. However his career was ended abruptly on the 1% August 1945 when riding a s tage of the G.P dy Debarquement du Sud ( just at the end of the war). Enter ing Marseille at the end of the stage he was struck by a jeep driven by som e American servicemen, in the city to embark on their ship back to the Stat es. Rushed to hospital his damaged leg was amputated that evening. he survi ved the operation and lived to the age of 80, dying in 1992.
His climbing ability helped him to 10 place in the 1938 Tour de France and 11 spot the year later. He was a prolific and very popular rider, with a gr eat sense of humour, an attribute that made him popular in the peloton.
Somewher..I have a brochure about this gear!!!
Norris Lockley, Settle UK