This is a sword bayonet manufactured from the period of 1856 to about 1878. They are typically British in design in that it has pressed-leather grips, "cock's-comb" upper finial, will lower "button" finial.
These are attached by fitting the "muzzle-ring" over the barrel muzzle, and fastening the pommel to a bar (lug) on the barrel or lug on a steel band. This example attaches to a bar on the barrel. The main difference between the the Pattern 1858 and the 1856 is: the 1856 attaches to a bar (or lug) on the barrel and has a muzzle ring in which the inner diameter is in line with the tang of the blade; the 1858 attaches to a bar on a barrel-band and has a muzzle ring in which the inner diameter stands off about 1/8" above the blade tang (about 1/4" above the blade back edge).
Metal furniture is iron (steel); latch is of external spring-steel type with integral push-button; grips are of black pressed-leather.
These were designed to fit the .577" caliber Enfield "2-band" Rifled Infantry Short-Muskets of the period.
Blade length is about 23" and muzzle ring diameter is ~7/8" (internal).
Scabbards were black leather with steel furniture (throat and drag).
These bayonets were popular with North and South during the American Civil War.

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