Outsider is the brand of Établissements Charles Maillard et ses Fils, a company which may have been based in Badevel, near Montbéliard, in the Doubs region in eastern France, close to the Swiss border. I don't think that the company had anything to do with the famous Maillard company based in Incheville, at the other end of France, which was an important manufacturer of hubs and freewheels.

The first evidence for Outsider derailleurs dates from around 1935, and there was definitely an Outsider Leger 42 model, that, I would assume, dates from 1942. I also have a copy of an invoice dated August 1945, indicating that the brand survived the second world war in some form.