Simplex Plunger - push-rod derailleurs
Lucien Juy was a bike shop owner who created Le Simplex derailleur in 1928 and got them into competitive use in 1931. Compared to other mechanisms that had a separate metal guide to move the chain left and right and a separate chain tensioning mechanism, this combined both functions into a compact unit. Once the metal guide was replaced with an upper pulley, it took on a form that resembles the modern parallelogram derailleur.
Steve in Peoria