This Springfield Armory manufactured first variation breech block with the thin firing pin wall is stamped "Model / 1873 / (eagle head / crossed arrows) / U.S." and is for the model 1873 trapdoor rifle. This breech block is in good condition exhibiting a grayish brown patina possibly having traces of mellowed original case colors and having a few specks of rust and/or pin prick pitting.  (file photo)

Note: Trapdoor rifles and carbines manufactured before March of 1874 and having serial numbers below circa 18,000 would have been manufactured with the first variation breech block.