Semper FI
The method of making a dual 45 cylinder by recessing the inside portion of the rear cylinder face and leaving a ring of metal around the outside edge for rimmed cartridges will not work using 1/2 moon clips. As they ride on the outside edge. Op stated he had used 1/2 moons. 455 ammo has an even thinner case rim than 45 colt, so chambering those in a 45 acp cylinder would give you huge head space. Then the statement that the serial number is visible on the rear cylinder face.
Makes me confused. Possibly a 45 acp cylinder with it's serial number and the 455 are head spacing on the cartridge mouth. The over all on an acp case is .012 longer and the chambers then that much deeper, but it would probably fire the 455s

Edit. Did some research and math. If you used the shorter MKII ammo and it's rim .04 sit on the cylinder space you would have .044 more head space than an acp. A dime is .053, so if the hammer nose protruded that much and there was minimal endshake, it would probably fire the 455 rounds. Really give the cases a run at the recoils shield, but then the 455 isn't exactly a power house.

Does the cylinders serial number match the one on the butt of the gun? I don't see how it can if marked 455 and it fires 45 acp in 1/2 moons.