According to Hiroshi Nakamura a flowering of sports bikes in Japan in the mid 1950’s was caused by a combination of the 1949 film ‘Aoi Sanmyaku’ about rebellious youth overcoming fuddy-duddy conservatism (and riding bikes while doing it), and a realisation by the bicycle industry that car sales were growing (and therefore that bikes needed to be repositioned as ‘leisure items’).

The result was a brief cycling boom in Japan from 1955 to 1958 - and a number of manufacturers started to produce pull-chain derailleurs that were closely modeled on European designs. Shimano was a follower of this trend, not its leader, and produced two designs, loosely derived from Cyclo models in 1956 and 1957 (Sanko had product from 1947 and Suntour from 1955). In 1958, when the boom ended, SunTour and Sanko doggedly carried on making derailleurs but Shimano quickly ceased their production and (very wisely) concentrated on hub-gear products.