This slightly mysterious object appears to be a Simplex 303 Tourist (see French Registered Design # 50765-001), but with a slightly different mounting arm designed for a rear opening dropout. Rear opening dropouts were much favoured in Japan, on track/fixed frames, on utility bikes and possibly on the ‘vélomoteurs, motocyclettes, scooters’ mentioned here.

• Publisher: L’Institut national de la propriété industrielle
• Inventor: Lucien Juy
• Country: France
• Date: French application February 1955
• Derailleur brands: Simplex
• Derailleurs: Simplex 303 Tourist?