http://en.wikipedia.org/wiki/Webley_Revolver
Mk IV: The "Boer War" Model. Manufactured using much higher quality steel and case hardened parts, with the cylinder axis being a fixed part of the barrel and modifications to various other parts, including a re-designed blast-shield. Officially adopted 21 July 1899, with a 4-inch (100 mm) barrel.[21]
Mk V: Similar to the Mk IV, but with cylinders 0.12-inch (3.0 mm) wider to allow for the use of nitrocellulose propellant-based cartridges. Officially adopted 9 December 1913, with a 4-inch (100 mm) barrel, although some models produced in 1915 had 5-inch (130 mm) and 6-inch (150 mm) barrels.[22]
Mk VI: Similar to the Mk V, but with a squared-off "target" style grip (as opposed to the "bird's-beak" style found on earlier marks and models) and a 6-inch (150 mm) barrel. Officially adopted 24 May 1915,[23] and also manufactured by RSAF Enfield under the designation Pistol, Revolver, Webley, No. 1 Mk VI from 19211926.[24]
So I'd say if the VI was used in 1921 then your V would be between 1913 to 1921.
Deaf