The letter of authenticity mentioned above can be requested by the information in this link:
Smith & Wesson Historical Foundation - Letter Process - Insuring that the rich history of Smith & Wesson will continue for generations to come
Historically, the British contracted with S & W and many other companies to provide sidearms in WW I in their standard service cartridge. The first model S & W provided was the Triple Lock, which had design features that might cause it to jam more easily with mud and debris, so the second model deleted those.
Several methods exist(ed) to convert the .455 cylinder to .45 Colt; the best way was to lengthen the charge holes, slightly counterbore the chamber mouths and thin the recoil shield to fit the thicker Colt brass rim. If this was done with yours, you should still see the serial number on the rear cylinder face.
Enjoy and treasure that gun!