This is the first patent for a Spirax derailleur. It’s a mysterious design, the cage of the single pulley travels along a spiral groove. This spiral moves the pulley backwards and upwards as it moves inwards. To me this is counter-intuitive - surely it would maintain a more consistent chain gap if it moved the pulley cage downwards and forward.

The drop out fitting plate also looks particularly odd.

Raymond Bon’s address is given as Courbevoie in the shadow of La Defense, Paris.

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• Publisher: L’Institut national de la propriété industrielle
• Inventor: Raymond-Eugène Bon
• Country: France
• Date: French application October 1935
• Derailleur brands: Spirax
• Derailleurs: unknown