This is a socket bayonet manufactured from the period of 1873 and saw limited use. They are extremely interesting and unique, if not ingenius.
These are attached by fitting them over the barrel muzzle, and fastened into position by means of a bridge and mortise slot with integral locking collar.
All metal construction, "blued finish."
CAUTION: Watch for spurious alterations to modern reproductions!!!
These were designed to fit the 45/70-caliber 1873 "Trapdoor" Springfield Rifle and its variants. The intended use was to attach this implement to the end of the rifle and use as an entrenching "trowel" or shovel or perform various tasks that required digging. It was obviously intended as a weapon as well.
Scabbards were leather with brass drag (tip); circular brass U.S. "rosette" is attached to swivel on frog/belt loop.

Please Note: These are currently being reproduced and offered by various distributors. Beware of these being offered as originals by secondary sources.