This is a wild and wacky design that is almost less of a pulley wheel based derailleur and more of a Simplex Selection fork derailleur - but with the chain tensioner attached.

I think that it never made it into production - but if your grandad has one in his loft please feel free to prove me wrong.

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• Publisher: L’Institut national de la propriété industrielle
• Inventor: Lucien Juy
• Country: France
• Date: French application September 1937
• Derailleur brands: Simplex
• Derailleurs: unknown