Congratulations, kcub. Do you mean that it was converted to .45 Colt or .45ACP?
As Petah indicated, those .455/.45 guns can be confusing. A number of the .44 HE 1st Models (less than 700) were converted to .455 for the Brits. Very few of the originals (less than 25) were originally made in .45 Colt.
Additionally, the .455 Mark II HE 1st Models which were originally chambered that way for the English were manufactured in a different serial number range from the 1st Model .44s so it is possible that there is some overlap between the numbers in the two models. Both of them might be found with the cylinder modified for the .45 Colt.
Let us know the serial number when you get it and we may be able to more clearly ID it for you. If you can get us close-up pictures of the markings and the back of the cylinder that will help.