FWIW, S&W apparently added the "U.S. Service CTG's" in hopes of obtaining a government contract. They dropped it c. 1909 when the .38 was no longer the standard U.S. service cartridge. Even prior to that, not all barrels had that marking and there seems no rhyme or reason for its inclusion/omission.
The reported marking on the OP's gun seems so unlikely that I concluded that only a Spanish revolver would be marked like that. Other common markings are "USE CARTRIDGES THAT FIT BEST", "USE SMITH & WESSON CARTRIDGES" with the end words in very small letters, and one of my favorites "THE KING AT ARMS".