A bayonet stud is typically on the rifle, and a slot (Mortise) on the bayonet handle to accommodate the stud. Plus the stud looking piece on the OPs find would be too big to be a bayonet stud.
I'm not seeing signs of a socket, to imply it's a socket bayonet.
No suggestions as to what it is or was.
The veins or striations in the metal, leads me to believe the piece is wrought iron. While there were some bayonets that had wrought iron parts not many had a wrought iron blade.