Here is a better chart with the date ranges on it. On shotguns Birmingham used the second in the string of marks as the provisional black powder proof. I don't recall seeing that mark anywhere other than the first inch or so of the barrels.
edit: Not that it should matter on your British gun, but the French (St. Etienne) black powder superior definitive proof mark, the German shotgun proof mark, and the Danish proof on Sjogren shotguns were all crown over S at different times.
|