Quite a task and I hope you'll post pics when you finish the project. Just purely a guess and off the top of my head I'd say it was made by Charles Francois Galand of 13 rue d'Hauteville, Paris along with A. Sommerville who held several patents on revolvers together:
http://www.google.com/patents?id=2eJOAAA...cad=0_0#PPA2,M1 . Maison Galand, possibly his son, was making doubles at 280 rue d'Hauteeville in the latter part of the 19th century. But let me check for other possibilities in the 1824-1856 period when the "crown" over "crossed palms" was used at the St. Eitenne proofhouse.
The "40" in the oval is the number of lead spheres to the French pound which would have equated to a 16 bore in British measurements. Is there another "S*G" below the "S*G" that is visible? Are there any stamps on the backside of the locks?
Kind Regards,
Raimey
rse