You have a .38 Hand Ejector, Model of 1902 Military and Police revolver. Its SN indicates it probably shipped in the 1902-03 period. The "U. S. Service CTG" part of the barrel caliber stamping refers to the .38 Long Colt cartridge, the standard military cartridge of that time. This cartridge pre-dates the .38 S&W Special cartridge, which is slightly longer. But in this revolver, either one can be used. The .38 LC is now obsolete. It's recommended that if you chose to fire it, it is best to use only standard velocity .38 Special cartridges with lead bullets.