This sounds like a Colt New Service DA Revolver, made in .38 special, .357 mag (intro. 1936), .38-40, .44-40, .44 Russian, .44 special, .45 Auto, .45 long Colt, .45 Eley, .455 Eley & .476 Eley.
Six-shot cylinder, bbl lengths 4-, 5- & 6-inch in .38 & .357 calibers, 4.5-, 5.5- & 7.5-inch in other calibers; 9.75-inches overall length (w/ 4.5-inch bbl.) Weight: 39oz. (45 cal. w/ 4.5-inch bbl.). Fixed sights, Blued or nickel finish. Checkered walnut grips. Made from 1898-1942.
Note: more than 500,000 of these in 45 Auto caliber (designated Model "1917" Revolver) were purchased by the US Government during WW1. These were later sold as surplus to NRA members through the DCM.
Commercial model......... NiB $1619 Ex $1295 Gd $881
Magnum........................ NiB $1031 Ex $825 Gd $561
1917 Army.................... Nib $994 Ex $795 Gd $541
(from 26th Edition Gun Trader's Guide)
Mike