I was hoping someone who actually understood British proof marks would confirm, but IF the gun was pre-1887, and the barrels were choked, it would be marked (usually on the tubes just forward of the flats) with the bore (in your case 13 or 13B) and muzzle constriction (ie 14M) and NOT FOR BALL. Are the barrels choked?
It was definitely proved again 1896-1904 with a bore of 13/1 = .719" - .728" for 1 1/8 oz. MAX m

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