1960s vintage Lyman Manuals have loading data for both the longer Mk I and the shorter Mk II cartridges.

Old manuals are usually available at gun shows if you look for them.

Also, Pet Loads by Ken Waters has a lot of very useful information and load data on the .455 Eley/Colt/Webley.

By the way, if you are interested in the history of this cartridge "family", Cartridges of the World is a good place to start.

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