frobee50,
Welcome to the Forum.
Check out this thread:
Let's see your U.S. Model 1917 S&Ws!
Also, do a search for "1917" and you will get a ton of threads.
Basically, when WW I started, the Brits didn't have enough Webley revolvers and they bought .455 revolvers from Smith & Wesson. In 1916, D B Wesson patented the half moon clip, which enabled the rimless .45 ACP round to be extracted from a revolver.
When the US entered the World War, we did not have enough 1911 Colts to equip the greatly enlarged armed forces. S&W and Colt were building large frame revolvers and both built 1917 revolvers. Since S&W had already been building .455s for the British, it was very easy for them to merely forge barrels in 5 1/2" length instead of the British standard 6 1/2"