It looks like it was converted to 45ACP(moon clips only)/45 Auto Rim. Normally, in an unconverted .455, the gap between the rear of the cylinder and the recoil shield is only about .045”-050”. Once it has been shaved, it is an obviously larger gap of .090”-.095”.
A purpose-built ACP/Auto Rim cylinder has a sharp shoulder in each chamber for ACP cartridges to use for headspacing. It provides definitive contact at the case mouth, just like in a 1911 barrel.
The .455 Eley chamber runs a bit deeper than the case length for ACP/Auto Rim. So, when it is converted there is no shoulder, just the taper ahead of the case mouth before the cylinder throat per se. And, generally all that is needed to perform the conversion is to shave the cylinder face by 0.045”.
A great way to enjoy these .455 converted revolvers is to load your own cartridges using 45 Auto Rim brass and cast lead bullets sized about 0.455”-0.457”.
With the right powder charge you can exactly match the original 455 ballistics. Plus, the loaded cartridges will look very close to an original 455 cartridge.
Attached is a picture I took last summer. While travelling abroad, I acquired a box of CIL Dominion 455 Colt cartridges. These were loaded in Canada for use in Colt New Service and S&W 455 revolvers. The safest way for me to get these home was to break them down to the components, then take home the primed brass, bullets, and box.
The next three pictures compare a 455 Colt CIL Dominion brass case (left) to a R-P 45 Auto Rim brass case (right). You can compare case length (nearly the same) and rim thickness (1st image). Note that the 455 case is a folded “balloon” head type while the Auto Rim is the much thicker and stronger solid head type (2nd image). Both cases use a large pistol primer (3rd image. Note: some other 455 cartridges use small primers).
