It should be possible to determine when the revolver was purchased from its serial number. A friend has one in this condition in .45 (not .455) Colt from a batch purchased in 1942. With the walnut stocks and silver medallion, this revolver would be later than NWMP or RNWMP periods. The MP stamp would confirm this.
I don't think that the value of a WWI service revolver would particularly relate to the value of an MP issue revolver. Named WWI officer's pistols, both Colts and S&Ws, are relatively more common than MP marked ones.
FWIW, I can remember when ex-Mounted Police service revolvers were being sold by a dealer in NY State, Hudson's Sporting Goods - this would have been well over 40 years ago. You had a choice of NWMP, RNWMP or MP marks, $24.95 or $29.95 depending on condition.