Re: Webley Mark I
Deadin, I think your first gun is a Mk I with a barrel from a later Mark. Mk I's were not upgraded to the new cylinder retention system but the barrel of a Mk III would fit a Mk I if the frame were altered to take the lever fixing screw. The second picture is of a Mk II; it is the same as the Mk I except that it has no hump on the backstrap. AFAIK, all the service models Mk I, II and III were made only with the 4" barrel. Both the guns pictured have been "shaved" to allow the use of .45 ACP with half-moon clips.
The Mk I was made with the recoil shield as part of the frame; the Mk I* had the shield as a separate part dovetailed in with a screw to keep it from moving sideways.
The Mk IV (not to be confused with the Webley commercial Mk IV in .32 and .38) was approved in 1899 and was the same as the Mk III but with a slightly different hammer spur and a sturdier firing pin. It was also available in 5" and 6" barrels in addition to the standard 4".
The Mk V had a slightly larger cylinder (.012" larger) to make it safe with smokeless powder. (see note)
The Mk VI, adopted in 1915, is the "Webley revolver" that most American gun hobbyists know, as thousands were sold war surplus in the 1950's. It is the quintessential "British army revolver" and is seen in just about every WWI and WWII movie. While the frame is not very different from the Mk V and previous Marks, the longer barrel and square butt make it look larger than the earlier models.
Note: I strongly advise against firing ANY of those converted revolvers with standard .45 ACP or .45 Auto Rim. The pressures of factory .45 AR are lower than the .45 ACP, but still way above the standard pressure of the .455 Webley. It is of interest that the British War Office issued a bulletin warning against use of the .455 Webley Automatic cartridge in ANY of the service revolvers, so the danger of high pressure loads in those revolvers is not something some American dreamed up last week.