This is Raymond Bon of Spirax's second patent for a derailleur. It’s a development of the design featuredg in French patent # 796,583. Again the cage of the single pulley travels along a spiral groove. Again this spiral moves the pulley backwards and upwards as it moves inwards. And again this is counter-intuitive as it would maintain a more consistent chain gap if it moved the pulley cage downwards and forward. However by 1939 Raymond Bon had added an extravagantly shaped pusher plate that may have kept the chain gap more even.
This basic design, including the spiral movement and luxurious pusher plate, was produced by Chaluret under the model name 'Chaluret 93'. Raymond Bon himself rode the 1944 Critérium de la Poly using a Chaluret 93.
Other notable features of this patent are: