This April 1937 advert contains a trove of interesting pieces of information:

• The Huret range is now composed of two models, the Huret Course Pro and the Huret Touriste. It is not clear what differentiates them.
• These can be fitted to the dropout, without clamps or braze-on fittings.
• One of these models weighs 250g (presumably the Huret Course Pro) - which is not shabby, even by today's standards.
• They are the only derailleurs trustworthy enough to be used for cyclo-cross.
• Fascinatingly, they appear to be manufactured for Huret by Chaluret.

This derailleur seems to be operated by a rigid cable that can transmit torque, a bit like the ones later used by Huret to drive its speedometers. This cable appears to be twisted by some unseen control lever to turn a cog that is mounted on its end which, in turn, drives a toothed, sliding, shaft in and out. The pulley cage seems to be mounted on the end of this shaft.

The relevant patent is French Patent # 813,241 which includes some clearer drawings.

(Source gallica.bnf.fr/ Bibliothèque nationale de France)

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• Publisher: L'Auto
• Country: France
• Date: April 1937
• Derailleur brands: Huret, Chaluret
• Derailleurs: Huret Course Pro (1st style), Huret Touriste