This is a socket bayonet manufactured from the period of 1853 to about 1870. They were in service - in some form or fashion - until about 1875. They are typically British in design in that they have a blade "shoulder" and are shallow fullered, whereas American bayonets of the period were without a shoulder and had very deep fullers that extended out the back 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 .577" caliber Enfield "3-band" Rifled Infantry Muskets of the period.
On this specimen: blade length is about 20-1/2" and muzzle ring diameter is ~0.79" (internal); socket length is ~3".
Scabbards were leather with brass throat and drag; leather body is without a sheet-metal - or wooden - internal liner as is commonly found on later British-pattern socket bayonet scabbards.
These bayonets were used extensively - both sides - in the American Civil War.

Some differences in common British socket bayonets:

• The 1853 "3-Band" Enfield Socket bayonet has a common blade-length of 17" to 18" (although longer has been noted on American specimens); the socket internal diameter is ~.781"; the socket-length is ~3". This bayonet - when attached - rests to the right of the barrel-muzzle. The "sight-bridge" is in-line with the blade.
• The 1876 Martini-Henry Socket Bayonet has a blade-length of ~22"; the socket internal diameter is ~.718"; the socket-length is ~3". This bayonet - when attached - rests to the right of the barrel-muzzle. The "sight-bridge" is in-line with the blade.
• The 1895 Martini-Enfield Socket Bayonet has a blade-length of ~22"; the socket internal diameter is ~.656"; the socket-length is ~3". This bayonet - when attached - rests under the barrel-muzzle. The "sight-bridge" is not in-line with the blade, but at 90 degrees to it (this is the easiest way to identify the Pattern 1895).