The Model of 1899 Smith & Wesson 'Military & Police' Revolvers, appeared in 1899, but, may not have been chambered for the .38 S&W Special Cartridge immediately, even if, or even though, the Cartridge was in effect born, in 1899 or possibly in 1898 even...so, there is some ambiguity as for maybe when the Cartridge made it's debut in available Chamberings and in Off-the-Shelf Boxes of Cartridges, and, now that we get down to it, I am not sure when that was, if it was 1899, 1900, 1901, or, as some sources say, 1902.
But I do have at least three S&W Revolvers, chambering the "38 S & W Special or U S Service Cartridge", which were made fairly early on.
I will have to ask Radagast for an evaluation of their years...and, also, if he has any strong opinion on when the .38 S&W Special Cartridge Chamberings first became available.
My own impression, was that the .38 Special Cartridge chambering, and, Cartridges to chamber in it, were available in 1899, even if some of the S & W 'M&P' Revolvers were intended for U.S. Service Cartridge ( .38 Colt ) only, as suited the Military at that time.