This is the second patent for a Spirax derailleur. It’s a development of the design featuring 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 Spirax had at least added a shaped pusher plate that effectively keeps the chain gap more even.
Other notable features of this patent are: