It may have been in "American" service, though. Looks like a Lorenz rifle bayonet but without a little retaining clip.
Not my pictures.
Lorenz rifles and their bayonets got into the US two ways. First, Lorenz rifles were probably the second most common foreign rifles in US and CSA service during the Civil War. Second, tons of surplus rifles and bayonets came in after the Austro-Prussian War of 1866.
A lot of the Lorenz rifle bayonets appear to have a more diagonal cut in the socket body, but Lorenz rifles and presumably their bayonets were made by a large number and variety of subcontractors. I'd look into Lorenz rifle bayonets, I think that's what this is. The "long s" stamp in front of the "8" also leads to one of two conclusions: Germanic or early 19th Century British. "ER" could be "Edward Rex" for Edward VII or VIII of Britain, but that bayonet looks like something for a muzzleloader, rather than say a .303 Martini, so I think "ER" isn't an English abbreviation.