Excellent info Dave. Saint Etienne was part of Vichy - Riaille located in Loire-Atlantique department was not. Since the retailer's name is on the gun, I suspect it was confiscated in Loire-Atlantique Department and sent to Germany from there. Riaille is a small place but is an agricultural cross-roads and it would be logical to have had a hardware store/armurerie there in the 1930's. A search of ads in the newspapers of the region in the 1930's would probably turn up some advertisements of sorts.

Might"J.G." may Jean Gaucher?; He was well known in Saint Etienne located at 12 rue Docteur Cordier: Here are several advertisements from Naturabuy.fr for a Gaucher fusil de chasse - unfortunately French sellers on Naturabuy are bad photographers it seems - only one picture of the underside of a barrel and it's so blurry you can't confirm whether it has "J.G.":
https://www.naturabuy.fr/Fusil-gaucher-item-4532732.htmlhttps://www.naturabuy.fr/Gaucher-Cal-12-70-Fabrication-St-Etienne-item-4445554.htmlhttps://www.naturabuy.fr/fusil-juxtapose-item-3229546.htmlhttps://www.naturabuy.fr/juxtapose-gaucher-item-3058576.htmlhttps://www.naturabuy.fr/vends-fusil-GAUCHER-mod-Tarzan-cal-16-70-item-4252111.htmlGaucher has been mentioned before - hewas in Business in Saint Etienne beginning in 1837....sometime in the 1970's he combined with Bretton and the firm is now known as Bretton-Gaucher. His guns from about 1950's on were marked "Manufacture Stéphanoise d’Armes J . Gaucher, St. Etienne: Don't know what the marks were pre-WWII or what the marks were if he made only the barrels.

Here is a bill of lading from 1909:

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