Patience while I put on the "old professor" hat and go into lecture mode.
One of the problems faced by early double action cartridge revolver designers was preventing the cylinder from free rotation when the trigger was released or when the gun was put back in the holster. If the cylinder rotated back, it could result in in the next trigger pull dropping the hammer on a fired round; if it rotated forward, it could miss a live round and the shooter would have a round less than he though he had. Either situation could be a nasty surprise in combat.
The problem was not acute in most solid frame revolvers, because the firing pin stayed in the fired primer until the hand picked up the cylinder ratchet for the next shot. But it became critical when rebounding hammers were used, either for safety reasons or to allow the gun to be opened, or both. With a rebounding hammer, the cylinder had to be kept from rotating backward or forward when the gun was at rest.
In the U.S., the Colt Model 1889 Navy had that problem and it was the reason the Army did not adopt that revolver until Colt corrected the problem in the Model 1892. Colt also used a second cylinder notch for an auxiliary cylinder stop, though the way it works is different from the Webley.
Since the second cylinder stop had only to keep the cylinder from rotating when the gun was not cocked and allow the hand to re-engage the ratchet, it did not need to be very precisely fitted, and both the Colt and Webley systems allow a fair amount of slack, though the Colt has less than the Webley.
The design used by S&W, Ruger, and the Enfield No. 2 revolver lets the cylinder stop remain in the cylinder notch for the last fired chamber when the trigger is released and the hammer down. The other Colt double actions, after about 1907, do the same thing, in a different way. Those guns don't have the free rotation problem.
Jim