Your Mark III is definitely antique and you have the RCMP paper to prove it. There is a service that researches Webley history and you can find out a lot about your revolver from them. (there is a charge for it) I recently donated a Mark V that was traced to a private purchase by Lt. H.A F. Gibson 58th CEF to the Royal Canadian Regiment museum. I have a Mark VI that I use occasionally at the range with RCN marks from 1916. If it is still a .455 you can have .45 Colt brass machined down to reload for it. Don't use .45 acp factory ammo, get mild lead reloads for safety. Enjoy the Mark III.
"Some commercial models of the Mark III in calibre .455/.476 were produced with either a 4" or 6" barrel. Another option was to have a squared butt instead of the birds head grip, this was similar to an option on the WG models. A separate, reshaped frame casting was used to accommodate the flared grip. Serial numbering did not continue on from the Mark II, the lowest observed number was 101 and the highest being number 80012. The Mark IV was introduced in summer of 1899.