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Info on moderated group rec.guns is at http://doubletap.cs.umd.edu/rec.guns
This is one of those cartridges with many names. The Manual of Pistol and
Revolver Cartridges Vol. 2 by Erlmeier-Brandt, page 228, gives the following
as synonyms:
.455 Revolver C.F. a.k.a. .455 II, .455 Colt Mk II, .455 Eley, .455 Revolver
Mk II. Adopted 1897. This is the chambering of your Colt New Service. It
does not list .455 Webley as one of the synonyms. However, on page 272 of
Cartridges of the World 8th Edition, "This is the present 455 Webley
familiar to American shooters."
White and Munhall, Pistol and Revolver Cartridges, page 112, list the
following as synonyms: .455 Revolver Mk II (also MK III IV V VI), .455 Colt
Mk II, .455 Eley, .455 Webley Mk II.
So after all of that, I would say the cartridges and guns have the same
chambering.