Based on your description of the Proof marks on your gun they
appear to be Birmingham, Nitro proof in use 1904-1925.
1 1/8 ounces is the maximum shot charge to be used in the gun; always provided that the approved breech pressure of 6720 p.s.i.is not exceeded.
BP BV NP and the words nitro proof are part of the approval markings applied during and after proof testing.
12/C indicates a gun with chambers that can vary in length from 2.5 -3.0 in length. Almost certainly your gun started life with 2.5 chambers which is supported by the 1 1/8 shot load marking.
12 is the diameter of the right barrel at proof -.729[nominal]
13 is the diameter of the left barrel at proof.- .719[nominal]
The right barrel is cylinder.[No choke designation]
The left barrel is choked ,usually when new, this quality of gun would be bored full choke, .040 constriction at muzzle diameter.
IT IS IMPOTRTANT THAT FOR SAFETY REASONS, BEFORE USING THIS GUN THAT THE BARREL DIAMETERS, CHAMBER LENGTH AND BARREL WALL THICKNESS BE CHECKED FOR SIZE BY A COMPETENT GUN SMITH.MANY GUNS OF THIS ERA HAVE BEEN OVER BORED AND CHAMBERS LENGTHENED RENDERING THEM OUT OF PROOF.
