Ah!, the .455 "family" of cartridges. More than likely what you have is a .455 Webley Revolver MkII which is a shortened version of the .455 Webley Revolver MkI. (The shorter cases were more efficient with cordite or smokless powder). The MkI was also loaded in this country as the .455 Colt and sometimes known as the .455 Enfield.
Confused yet? An earlier version was known as the .476 Revolver MkI which was a replacement for the .450 Revolver. According to Barnes these are all pretty much interchangeable although you aren't likely to run across any of the older ones.
Dominion (in Canada) loaded .455 MkI and MkII up until a number of years ago.
Fiocchi still loads MkII, but it is pricey. I've heard rumors of new brass being available, but can't point you in the right direction.
Lee Precision make dies at a reasonable price. .454 Long Colt 250-60 grain lead bullets work well. There is a "correct" 265 grain mold available, but costs an arm and a leg.

Dean

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Dean
SWCA #680 SWHF #446