I have a pre-1921 Clabrough & Johnstone, Birmingham shotgun with the same proof markings which date 1904-1925. I see in one of your photos that the barrels have a little crossed sword mark, this should give you the year it was made. Before 1921 Birmingham guns had no date marks, after 1921 they did. See the attached web link for the Birmingham marking dates. Very nice vintage gun.

www.shotguns.se/html/uk.html