Thanks for the additional pictures eagle80.
If your example was altered from the .455 cartridge to 45 Colt it looks to have been done by reaming out the cylinder chambers and by shaving the face of the recoil shield, the back of the frame where the rear of the cylinder abuts when closed. This is done because the 45 Colt is longer and has a thicker rim that the .455 MKII cartridge. There are two other ways to accommodate the thicker rim. One was to shave the rear face of the cylinder, which also removes the serial number from it. The third, and most desirable mode, was to counter sink the chamber rims and ejector star which would allow use of both the 45 Colt and the original .455 round. I see no evidence of either method on your gun.
I have a 1st model .455 HE "Triple Lock", that was altered to 45 Colt by shaving the recoil shield.
The file marks are very obvious.
There is another remote possibility. Looking at my copy of the Standard Catalog Of Smith and Wesson I see that there were a very small number of the early N frame hand ejector 2nd models that were chambered in 45 Colt, about 700 or so, and in the same serial number range as the .455 caliber guns supplied to the British and Canadians. Unless there are obvious signs of the recoil shield being shaved down, the only way to determine whether your revolver was originally made for 45 Colt would be to spend the $50 for a history letter from S&W.
But I really think you have an altered .455 2nd model.
If you plan to shoot that old warhorse you may want to stay away from higher powered ammo with jacketed bullets. Cowboy action loads with lead bullets should be fine, I reload easy stuff for my old Triple Lock, usually Trailboss powder behind the same 230 gr home cast lead round nose bullets I use for my 1917. Here are the two. TL on the left, 1917 to the right:
They both are a lot of fun. Enjoy yours!
John