Or quite possibly and not at all unreasonable a mid 19th century naval dirk.  I have been researching what bayonet this could have been made from if that is the case, and I have been unable to find one with the correct sizing profile and fullering.  The bold use of brass is also a very naval thing. ????  Interesting link.
http://asoac.org/bulletins/86_tuuite_naval.pdf