It is my understanding that the diagram posted by Baltimore Ed shows the SAAMI specs (maximum?) for the .455 Colt cartridge, which was effectively a North American commercial version of the Mark I Webley cartridge, although expressed dimensions do seem to vary slightly.
Here are diagrams from the British War Department's "Treatise on Ammunition", which give "high" and "low" acceptable dimensions. The Mark I diagram is from the 1902 edition of the Treatise, the Mark II diagram is from the 1905 edition -
For comparison, here is a composite of digrams from Dowell's "The Webley Story", for the Mark I and II bullets and the cartidge case used for the Mark I cartridge and the shortened version used in the Mark II cartridge (and subsequent cartidge versions) -