The 45 Webley cartridge was developed by American manufacturers (WRACO, UMC and Remington) from around 1875 to 1939. The 450 Short CF/Adams had been around since 1867 and was chambered in a variety of British and European revolvers and many ended up in the US but it was not until 1880 that US manufacturers began making ammo for the 450 Short CF. The Webley was thus loaded to supply the ammunition to the US market. The case is longer than the 450 Short CF case and contrary to popular belief was not underpowered. (Erlmeier, Brandt Ref. 439).
UMC 1910 Catalogue showing the 45 Webley loading.