I recently acquired a simple pull chain derailleur from Spain branded 'Izar'. I looks to date from somewhere between the late 1930s and the early 1950s. A correspondent suggested that this was likely to have come from the Basque country - the region that is home to both Zeus and Triplex, to a multitude of small precision engineering companies and to many of Spain's most fanatical racing cyclists.
A quick Google search revealed the existence of a company called Izar Cutting Tools S.A.L., based about 30km from Eibar (the home town of Zeus and Triplex). This company has the right kind of pedigree - but the history page on its website unfortunately makes no mention of manufacturing derailleurs. Despite this, my best guess is that, in some way, the antecedents of Izar Cutting Tools S.A.L. had something to do with the mystery derailleur.