This is a .455 Mark II, 2nd Model HE. There were .45 Colts that were shipped to the Canadian Govt. in Ottawa. So this can plausibly be a standard .455 or a factory .45 Colt. Let's focus on the details first before further speculation.
Let's start with this thread... please look at the recoil shield and photos of the chambers. The .455 chamber throat is approx. 65% down the length of the chamber from the rear, whereas a .45 Colt is more like 85%.
HE MKII 455 Cylinder Length
A .455 Cylinder is approx. 1.585" and a .45 Colt is approx 1.575", and a .45 acp is 1.54"
The headspace (between recoil shield & cylinder) on observed .455s is about 0.04" whereas observed 45 Colt cylinder headspaces is 0.06"... which is why a .455 Cylinder has to be modified or the recoil shield shaved; or both; or the ejector/chamber recessed.
Based upon these technical details, your photos indicate the current throat spacing in the chamber is for a .45 Colt. Most of us agree on that. THE Question is the head spacing for the rim of the .45 Colt...does it seat and does the loaded cylinder close with adequate space?
In my opinion, the OPs Revolver does not give definitive evidence of modifications from a .455 to a .45 Colt. Visually it looks like a .45 Colt to me. He could obviously eliminate a lot of questions by conducting these measurements to confirm/deny. Also why not give us a photo of a .45 Colt chambered in the cylinder (open and closed), closed to see head-spacing...or have you even tried to chamber a .45 Colt and close the cylinder...it will close or it won't.
Based upon answers to the above technical details we can dive into more subjective data regarding the serial number, shipping dates/records, and other comparable guns. The .455 Canadian/British/Commercial guns are very confusing when it comes to ship dates, duplication of serial numbers, proofs, etc. For example, there are guns that were not caliber marked and only had Smith & Wesson on the left-hand side. It is also rumored that the factory .45 Colts had a taller front sight when compared to the .455 sight, but I don't know the measurements or if this is factual.
So let's focus on the technical details and caliber prior to getting into these secondary details?
P.S. I'm in the same boat, currently waiting on my letter to confirm a .455 Mark II, 2nd Model HE .45 Colt that I own. Mine has been converted by King into a King Super Target but has the hallmarks of being a factory .45 Colt as based upon the above measurements.