Don't worry about that Webley cylinder not locking up when the action is at rest. The cylinder won't rotate, but it will move quite a bit out of alignment because it is held only by the trigger stop (the actual cylinder stop is on the trigger), which is narrow but has a wide notch in the cylinder. The design was actually outdated when the Mk VI was made, but Webley apparently never saw the need for a complete overhaul of their design.
Thanks, Jim, for the correction. I was a bit confused because the Mk V didn't jibe with the 1917 date. The Mk VI is stronger than the Mk V and previous Marks, but there is still too much risk of damage for firing factory .45 ACP.
Actually, the supply of .455 ammo in the U.S. never "dried up"; it was never there in the first place. Unlike the .45 ACP and the 9mm, the .455 was never used in SMGs or any guns other than the revolvers which were considered obsolescent in 1940. Plus the British never considered any handgun to be a real combat weapon; ammunition was issued for both the .455 and .380 on a basis of 12 rounds per gun, with an additional 12 rounds in unit supply. That was intended to last the war.
That meant that unlike the .45 ACP, which the U.S. issued with open hands, the British revolver cartridges were scarce and there was no vast quantity of surplus ammo. That was the reason importers converted those revolvers to use the .45 ACP. If they were aware that the cartridge was overly powerful for the gun, they obviously didn't let that bother them.