James you are correct this is a gun proof tested in Birmingham during the period 1875 -1887. There was a period of time in this era when Birmingham made guns,ordered for export were proof stamped on the barrels and not on the barrel flats. This practice was in place for a several years but was discontinued. In my opinion it was abandoned because the proof marks stamped on the barrel tubes rather than on the flats could act act as stress raisers weakening the barrels.I spent some time trying to identify the time frame when this practice was in vogue including discussions with the Birmingham proof house and leading figures in the Birmingham trade.I never obtained more than an aknowledgement that this was a practice used by some makers engaged in the export market.
This grade of gun boxlock none ejector is sold in provincial gun auctions in the U.K.for $100-200. The proof marks are black powder. The barrels a should be measured to see if they are in proof and to confirm the barrel material.
