A Mortise Chisel. This part of the mortise bit creates the square edges of the mortise. The drill bit that would hog out the centre of the mortise seems to be missing in this photo or is simply withdrawn into the centre of the chisel portion.
The mortise bit would be used in a drill press (with a mortise attachment) or in a dedicated mortising machine.
Brian
The mortise bit would be used in a drill press (with a mortise attachment) or in a dedicated mortising machine.
Brian