I'm thinking it's for an 1873 and later Trapdoor Springfield. I have one with the same measurements and markings. Also, I don't think the ones produced for the ACW were blued, but were finished bright.
Looks like a Trapdoor bayonet. Appears to be bluing and not rust. Some 1855 bayonets were cut down, an insert placed inside the socket and then blued.
These saw service through the end of the 19th century in both Indian Wars and Spanish American War. I don’t believe any were at the Boxer Rebellion.
As a note the last time the Trapdoor Springfield saw combat was in WW2 in the hands of Phillipino guerrillas.