Hey Indy, I see you are in Alberta. Possibly it was a "bring back" from WWI service by someone. You should find some martial markings on it, either tons of "proof" marks and stamps in numerous places if it was a surplus export from England, or very little if it was a "take home." Very, very few were sold commercially and usually not in .455 caliber. It should also have a swivel for a lanyard on the butt.
Also, a few hundred were used in RCMP service in the teens until ??? - but those were in .45LC to start, not modified. To tell if it's been modified, look at the cylinder face:
This one has been carefully modified to .45 Long Colt. There were two ways to do this:
1) rebating the cylinders slightly (about .015") to allow for the thicker rimmed .45LC cases to drop in. The original S/N is still visible on the cylinder face.
2) the entire cylinder face and ejector star could be machined down the same amount - your cylinder face will be either in the white, or reblued and have no S/N. This is also how they were usually modified for .45acp.
3) Also, determine if the cylinders have been lengthened to accommodate longer cases like .45LC - either you'll see some machining marks, or just drop test cases in and see what fits.
It's sort of a toss up - if its still .455 that adds collector value, but generally only in higher finish grades. If it is .45acp or .45LC, you will have a much easier time getting ammo and thus likely shoot it and enjoy it more. Use lighter loads of either the acp or LC ammo, this is pre-heat treatment days of manufacture by S&W.
Also, note that if it is converted to .45LC, you should hunt up the "old" .45LC ammo, like RickB pointed out. This was the bullet diameter standard before WWII - .454. This is the same as the .455 bullet diameter - .454". Smaller diameter .45acp bullets or modern .45LC bullets at .452 will be slightly undersized for the barrel. I believe most of these were converted in the 20s as I've seem them offered for sale at that time. Also, the vast array of British export proofs on them, which were applied to surplus guns leaving the country in "official" channels usually differ in some details vs. the Lend-Lease .38 S&W's done after WWII.
Your gun is commonly known as the "S&W .455 Mk. II Hand Ejector, 2nd Model" and was built in 1917 most likely. As Rick already said, the difference between the 1st Model and 2nd Model was the locking lug on the yoke/frame, and the full-length ejector shroud - these were deleted on the 2nd Model. People will just refer to them as a S&W "455" or "455 HE" - since no other S&W model was ever made routinely in that caliber (I think a few .44 HE's were, but very odd limited runs), there's no real possibility of confusion by just citing the caliber to indicate the gun.
If the gun appears is in good working order, you determine the caliber, and it can pass a "check out," I would not worry about test firing it with suitable ammunition. I enjoy shooting mine periodically as do many others owners I have communicated with.