The 1912 Le Chemineau derailleur was developed by Joanny Panel, based on a prototype built by Paul de Vivie. In many ways it defined many of the attributes of a modern derailleur. It has a guide pulley that moves in and out, moving the chain across a sprocket cluster, it has a sprung arm that provides chain tension with a tension pulley on it, and it was controlled by a single cable, working in opposition to a spring.
Eerily the centre to centre distance on the pulley arm is 75mm - a measurement commonly found today. The tension pulley has 10 teeth - another recurring theme.
Unfamiliar to us today are the ideas of mounting the derailleur on the underside of the chainstay, and of using a toggle chain to pull the derailleur along a rod - but both these ideas would have been totally mainstream as late as 1960.
This well used example (look at the wear on the teeth) is one of three in this collection. I think that this is an early example. Its pulley arm is oval in cross section and has a small hook at the top to take the spring.
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