This is the second of the patents relating to the definitive Spirax design (see also French patent # 934,933). The basic mechanism is basically the same as in the earlier patent, but this time it is mounted on a rotatable arm, so that it can be swung out of the way when removing the rear wheel. This is the classic configuration.
Note how neat, composed and self contained the derailleur looks. Given that it was also constructed to a very high standard (and sold at a very serious price), it was a classy piece of work.