This advert, which has little to do with derailleurs, features Peugeot’s amazing retro-direct transmission. Unlike the retro-direct machines offered by Hirondelle and Terrot, this system did not use any ‘extra’ pulley wheels. Instead it used a continuous double length chain that looped around two chainwheels and two fixed cogs at the rear, all on the same side of the frame. Each chainwheel had an independent freewheel ratchet between it and the crank. As with most retro-direct systems, you pedaled forwards for a high gear and backwards for a low gear.
(Source gallica.bnf.fr/ Bibliothèque nationale de France)