This Simplex Champion de France is an example of a design that Simplex produced in a lavish range of different variants and under a profusion of different names (Champion 35, Champion 36 and Champion du Monde being some possibilities) between 1935 and 1948. These derailleurs were a huge commercial success and did much to establish the derailleur as a standard piece of racing equipment. Simplex produced them in their hundreds of thousands, and marketed them by aggressively sponsoring leading riders to use them, establishing a pattern that persists to this day.
The Simplex Champion de France was a steel variant, with rather luxury chrome set off by fetching black touches.
None of this means that it worked very well. It was fragile, imprecise and had very limited capacity.
In the photograph I have disengaged the main spring to allow the gear to take up a more normal position.