Here is description of the Huret Super-As fork-type derailleur. It appears that you can tension and detension the chain by twiddling the knurled knob on the end of the lever that rises straight up from the bottom bracket.

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

• Publisher: L'Officiel A.C.M.
• Country: France
• Date: April 1935
• Derailleur brands: Huret
• Derailleurs: Huret Super-As