The gun I have (and thank you 105kw
) was "shaved" a bit less than many, so doesn't have the clearance others do. Works fine with the S&W half moon clips, also works fine with some 2rnd clips (maker unknown) I found at a gun show. NO full moon clip I have ever found works in that gun, most prevent the gun from closing, they are just too thick.
The clips were created originally for use in the Colt and S&W 1917 revolvers, to allow ejection of the rimless .45ACP cases using the ejector star. Without them, rimless cases must be poked (or picked) out of the cylinder one at a time.
Many (most?) Webleys got shaved when the supply of surplus .455 ammo dried up by the end of the 50s and the dealers dealing in the imported surplus guns had the work done, so people would still buy them. One doesn't sell many guns there is no ammunition for. The work was done by different smiths and there was no standard so, dimensions differ.
Remember, at the time, these were not rare collectible pieces of history, they were war surplus, obsolete for military use, common, and nearly as cheap as dirt.
The conversion to be able to take .45acp ammo was a marketing decision, and ignored the face that the .455 is a lower pressure round and firing standard .45ACP was essentially a proof load pressure for the gun. Some guns survived it well enough, some didn't.
Do be aware that the only Webley .45s that were rated to use smokeless powder (in the Webley cartridge) are the Mk V and the Mk VI, and production of the Mk VI ended in the 20s after WWI.