This advert was possibly placed in 1958, in an unknown Argentinian magazine, by a company called Cycles Motors S.A., based in Buenos Aires.
Cyles Motors S.A. was the main Argentinian importer of major Italian bike brands, Torpado, Wilier, Frejus, Roma Sport and several others, which is a pretty prestigious portfolio! It also imported a range of Italian cycle components and accessories. What Cyles Motors S.A. did not seem to import was any bikes, components or accessories from France.
So now we come to this mysterious advert. Firstly it shows a Simplex model that I have never heard of - a variant of the Simplex Juy 543 that is clearly branded as a 'Simplex Gran Sport'. Secondly 'Gran' is not a French word. And thirdly the gear lever pictured is clearly branded Simplex - but is also clearly a very close copy of a Campagnolo design.
What can we make of this? I would, based on no evidence, suggest two possibilities. The first is that this mystery derailleur was a model manufactured by Simplex's factory in Italy. The second possibility is that this was a model manufactured in Argentina itself.