There is generally no caliber marking on the barrels of the .455 MarkII Triple Locks. If the cylinder was shaved, the rear may still be in the white if it was done outside of the factory and there won't be a serial number remaining on it. It's also possible that a M1917 .45 ACP cylinder was installed, in which case, the number will be different but the back won't have been shaved.
It's unusual to see the .455 changed to a .45 ACP. The vast majority were bored out for the .45 Colt.