The dual caliber stamping on the barrel is a tipoff that it is either a Hand Ejector, Model of 1902 (round butt) or a Model of 1905 (square butt). It is known that it was probably first used around 1904, and stopped at around 1907-08 at a SN close to 133000. The "US Service Cartridge" is the .38 Long Colt, very similar to the .38 S&W Special, except it has a slightly shorter case. At the time it was the official US military revolver cartridge. A picture and the serial number stamped on the butt would be helpful for better identification.