Strange Calibre markings on Webleys
The .445,450..455 and .476 Chambers were all essentially the same. The cartridges differed in Construction, as to case length, bullet design (.476 was a "heeled" design). Ammunition could be used interchangeably to a greater or lesser degree...the case body and rim sizes were virtually the same. IE, one could safely use .450 Adams or Colt cartridges in a .455 or .476 marked gun, and sometimes vice versa. The differences in nomenclature was from the cartridge design, not really the gun chambering. The calibres mentioned were all cartridges used in Britain by Army and Police in the period 1870-1890s, finishing with the .455. Cases for all of them can be made by rim thinning and case trimming the standby US .45 Colt revolver case...it just depends on the load ( BP or Smokeless, lead or jacketed Bullet, Full diameter or Heeled).
There are about half a dozen case lengths to contend with, from the .445 Webley, thru .450 Boxer, .450 Adams, .450 Colt or Eley,.455 Marks 1 & II, and the Enfield .476 Army. There are also some Kynoch made .455 "Manstopper" ( Wadcutters) also.