Abilene is correct. The ..45-75 W.C.F. was the first to be introduced of the four 1876 calibers. All carrier blocks and barrels, were void of caliber markings until the introduction of the .50-95 Express in March of 1879. At that time, all 1876 carbines, rifles and muskets had calibers markings on both the carrier block and the barrel. The 1873 models were also unmarked for caliber until approximately serial number 51,000, when the .38-40 cartridge was introduced. An interesting side note, most people don't consider. The .44-40 (.44 W.C.F.) cartridge was the first center fire caliber developed and introduced by Winchester in 1873. The .45-75 W.C.F. was the second caliber developed and introduced by Winchester in 1876.