It is a Model of 1902 Military revolver, from 1902-03. That revolver will accept either .38 Long Colt ammunition (the standard U. S. military service revolver cartridge of the day) or .38 S&W Special. If you shoot it, it's recommended you shoot .38 Special wadcutter target loads. These are interesting and historic revolvers, and not too common. Your ammunition box could have some collectible value also.