Hi Scaleyback,
You must have quality woods out the back! See if you can find more out there.
There are lots of identifying features of the tank: the little roll down for the last 10cm at the back, the straight fold from the filler down to the lower corner, the two angles at the front with the short straight section that follows the frame head stock and the longer angle coming back to the bottom. Not to mention the unusual way its all soldered together.
I've just spent an hour flipping through 1920s French magazines and there are lots of similar-shaped tanks, but only Gnome et Rhone seems a likely match, around 1925. As R points out, one filler cap may mean two stroke engine (Gnome and Rhone had one in their range), but there is another possibility since G&R in this period had a separate oil tank on the saddle tube, so only a petrol filler on the main tank.
The attached advert is from La Revue Motocycliste et Automobile, 15 Juillet 1925. Also a photo from the interweb. Not sure exactly what model your tank might be off, but I suspect it will turn out to be Gnome et Rhone. Someone familiar with G&R (I'm not!) will tell you whether the unusual folds in the sheet metal are correct.
Cheers
Leon
