This is a socket bayonet manufactured from the period of 1873 and used to about 1900; it is very similar to its predecessor: the US Model 1855. They also saw service in state militias and guard units until about the end of WWI. They are typically American in design in that they do not have a blade "shoulder" and have deep fullers (or flutes), whereas British bayonets of the period had a shoulder and had shallow fullers that stopped at the shoulder of the blade.
These are attached by fitting them over the barrel muzzle, and fastened into position by means of a mortise slot and locking ring.
All metal construction.
These were designed to fit the 45/70-caliber 1873 "Trapdoor" Springfield Rifle and its variants.

• Blade length is about 18";
• Muzzle ring diameter is ~0.73" (internal);
• Socket length is ~3".
  

Scabbards were rolled sheet-steel with leather frog/belt loop; circular brass "rosette" is attached to swivel on frog.