Book on Clabrough will be out next year. Most Clabrough's I have seen that are marked 11 in proof are 10 gauges. The 11 is the bore size not the chamber size. Flintfan is correct. I estimate the the gun to have been made in 1875 and after the 1875 proof mark was in effect "NOT FOR BALL". I have in my study ser. # 1907 and it has the 1875 "NOT FOR BALL" stamp. This gun was ordered to be made without chokes. I would say it is a medium grade. most of their guns in this early period are: medium grade,are side lever opening, most have a wedge fore-arm fastener, a screw-in hex. nut firing pin, one locking slot under barrels and a steel butt plate. Ser. # 1907 also has the San Francisco address. Most have LONDON in the address. Perhaps these guns were sold through the S.F. store?
