This is but one of a multiplicity of patents for cunning chain tensioning devices driven by fiendishly clever combinations of springs and cables. I am not sure why - a simple spring would seem to suffice.

• Publisher: L’Institut national de la propriété industrielle
• Inventor: Lucien Juy
• Country: France
• Date: French application December 1935
• Derailleur brands: Simplex
• Derailleurs: Super Simplex