This Springfield Armory manufactured second variation breech block with the thick 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 very good condition exhibiting a mixture of applied hot immersion blue finish with a brown patina and having a few specks of rust.  (file photo)

Note: Trapdoor rifles and carbines manufactured between March of 1874 through June 1876 and having serial numbers between circa 18,000 through 70,000 would have been manufactured with the second variation breech block.