Note how the Tour de France 'ensemble' uses a twin lever on the right hand side of the downtube. But the secondary lever is not for detensioning, instead it controls the front derailleur.

(Source gallica.bnf.fr/ Bibliothèque nationale de France)

• Publisher: L'Officiel A.C.M.
• Country: France
• Date: June 1950
• Derailleur brands: Huret
• Derailleurs: Huret Tour de France