This enigmatic derailleur is labelled Izar Insuperable. It is a well engineered copy of a 1930s Simplex single pulley, pull-chain derailleur.
I have been told that this Izar derailleur was manufactured in the Basque region of Spain, probably in the early years of the Second World War. Trade between France and Spain had apparently broken down, and it is also possible that the intellectual property rights of companies from a defeated France were no longer rigorously respected in Franco's Spain (imagine!). The combination of these factors could easily have provided fertile ground for Spanish companies to manufacture copies of French derailleurs. This story is backed up by the existence of a Zeus patent for a rather similar copy of a Simplex derailleur. Zeus was, of course, also based in the town of Eibar in the Basque region.
The word 'Izar' has a, z-laden, Basque feel, and 'Insuperable' carries echos of a typically Basque determination to fight on, and on, and on.
Finally, and to further add to the intrigue, the inside of the main arm of this derailleur shows faint traces of lettering that is back to front and may spell out '...DEMARC H1... - EIB...'. Why this should be back to front is a mystery, but it is possible that 'EIB' is part of 'EIBAR'. Spooooky!