First, how certain are you of the identification of the pistol???
I'm no expert, will have to do some research, but if I recall correctly NONE of the early Webleys are safe to shoot with smokeless powder, and I know for a fact that NONE of them, including the last "modern" MK VI should be shot with .45acp ammunition!
They are pretty big guns, leading one to think they are strong guns, but they really aren't. Despite the fact that many (if not most in the US) were cut to take .45acp BRASS when the supply of .455 Webley ammo dried up, they should NOT BE FIRED with .45ACP ammunition!!!!!
Standard GI .45ACP ball is a proof level pressure load for the Webley. DON'T SHOOT IT in a Webley!!!
The guns can be shot using handloads, made in .45acp cases, but loaded to the lower pressures and velocity suitable for the Webley.
I have a Mk VI, dated 1917.
If you have a Webley Mk I, its black powder only, and if I remember right, would have been made in the 1880s or so. Will do some research and see just what I can find.
Again, even though SOMEONE cut it to take .45acp rounds DO NOT SHOOT .45ACP ammo in the gun. EVER.
and, did I mention, don't shoot factory .45ACP???