I've been told that the simplest way to tell whether a bayonet is a US Model-1863 (or earlier) Springfield bayonet or a Springfield Model-1873 bayonet is to see if a penny will fit into the bayonet's socket-hole, or not. The Model-1873's hole is smaller, because it fit onto a .45-caliber rifle's barrel, and the Model-1863 was for a .58-caliber rifle