P.Webly & Son merged with W.& C. Scott & Son to become Webley & Scott Revolver and Arms Co. on 21 October, 1897.

AFIK, no more guns (as opposed to Revolvers) were made under the P.Webley name after that. If so, that would put your rifle in a time-frame before that.

However, as Mickey noted, the provision for Nitro proof was not available until 1904, but it was not mandatory until 1925.

Accordingly, it is possible to encounter double rifles built to handle full nitro loads, but were only blackpowder proved in the intervening years. Must have been to save costs, but I have seen such rifles. The barrel contour and wall thicknesses will tell you whether it is for Nitro or BP.