I am sure the marking means ".45 Auto Rim" and was put on by the gunsmith who shaved the cylinder. Of course, any revolver that could take the .45 AR could also take the .45 ACP with moon clips.
Some notes on the .455 Webley Auto Pistol cartridge and the Colt pistol made for it.
That cartridge is semi-rimmed and is supported (headspaced) on its rim, not on the case mouth as is the .45 ACP. The Colt pistols (Government Models) made for the .455 Webley Auto are different from those made for the .45 ACP. For one thing, the barrel hood is grooved for the rim (like the .38 Super barrel), and there is no sharp shoulder in the chamber. In addition, the .455 magazine (marked ".455 ELEY") is wider than the .45 ACP magazine to accommodate that rim, so the magazine well is wider also. A .455 magazine will not fit in a standard .45 ACP pistol.
The .45 ACP can be fired in the .455 Webley Colt pistol (as well as in the .455 Webley pistol itself), but it is supported only by the extractor and performance and accuracy are poor.