Is the 38/200 Webley a No.4 MK2 or a Mk4 No.2?
I always get it mixed up.

If this isn't a joke, you're confusing the No. 2 series of .38's with the No. 4 MK II rifle!
Some refer to a MK IV rifle. There is one, but it's a .22 trainer. I don't know why Americans have so much trouble with this, but it seems very common.
The .38 revolvers were in the No. 2 REVOLVER series. Has nothing to do with previous numbering series or with rifles. The S&W was classed as a No. 2 but maker name added. I think they may have had more S&W's in WWII than Enfields. Not sure.
Modded Enfields were noted with one or more asterisks (*) to tell what had been done. Avoid No. 2MKI or II **. They removed the entire cocking notch on the hammer internally, and they can fire if dropped on the hammer. I think this mod was soon discontinued and guns re-hammered to MK I or II * level.
The MK IV.455 is numbered in the military series of official .455's. It has nothing to do with the COMMERCAL series of .32 and .38 revolvers.
The MK IV .455 appeared in 1899, just in time for the Second Boer War. The MK IV .38 was a 1927 item, Webley's version of what Enfield Arsenal had rejected. They put it on the market then, for commercial sales. It was an updated, slightly improved version of the previous MK III .38, with a larger grip frame and a few other differences. There was also a smaller pocket version, as with MK III.
In WWII, shortages of revolvers forced the govt. to buy Webleys. And Colt and S&W revolvers. I think Webley was still mad and maybe made those guns with rougher finishes than they might have otherwise. Just a guess. Anyway, they marked the rough ones as War Finish, and made them faster than normal. Commercial Webleys were normally nicely blued or nickeled.
Spare parts: I suggest buying mainsprings and stirrup lock springs and firing pins. You may never need them, but if you do, they're good to have on hand. I had to replace a mainspring and a hammer nose/firing pin on a MK VI. A gunsmith actually welded the firing pin to repair it. But I was a teen then and was probably doing too much dry firing. Most of these .455's are now over 100 years old and the steel may be getting brittle.