You actually have to be a little careful when shopping for a .455 Webley (assuming you would want such an ugly thing and want to go to the trouble of finding the ammo) that it hasn't been bored out.
Much of the .455 ammo still circulating is Berdan primed and can't be reloaded, but there's an outfit in the upper midwest that sells new brass turned down from .45 Long Colt. In addition to trimming the case, they grind down the rim until it's thin enough to fit. The cases have slightly thicker walls than the originals, which will mess up your loading data (start with mild loads!) but once you get that straightened out they are great.
So it may have even been a true .45 Auto as in .45 ACP, although that's a 1941 movie.
I don't think there's any danger of me wanted to buy one of those just yet, although I bought an old Lee Enfield on an impulse like that. You see a piece of history on sale going begging ...