The gun shows proof marks for 1 1/8 ounce nitro
As Jim pointed out, what we have to remember here is that our current 1 1/8 oz loads aren't the equivalent of the 1 1/8 oz load which was designated as the working load for your gun. Back when that gun passed proof, all British 1 1/8 oz loads were within certain pressure parameters. For American 12ga loads, current max pressure would be 11,500 psi, which is likely 20-30% higher than those old loads. Not to mention the fact that today you can buy 1 1/8 oz lead loads with velocities up around 14-1500 fps, so recoil will likewise increase dramatically. Neither of those things are good for old guns.
I've owned several British 12's proofed for 1 1/8 oz loads. For targets, I shot mostly 7/8 oz reloads in them, occasionally 1 oz, both loaded to very low pressures. I did use some 1 1/8 oz reloads on pheasants, but they developed just a little over 7,000 psi pressure.
