neophyte,
I was thinking it was more like this mortising fence. I was looking for a similar model for my bench top DP but haven't found one yet. The j-hooks hold material up against the fence and the center "foot" adjusts down to hold the material in place.
The OPs appears to be missing those j-hooks, or something similar. The center foot appears included and adjustable with the knob in the rear.
V/R
Bogie
The J hooks hold the material being mortised down to the drill press or mortiser table to prevent it from lifting when the chisel is lifted and removed.
The J hooks are in the first picture.
The adjustment rod is what the J Hooks are mounted on, so the J Hooks can be moved up and down for the thickness of the material.
The adjustment rod in this set up is part of the fence.
The fence gets mounted to the drill press table, you just manually slide the workpiece being mortised along the fence to make sure the mortise is straight and parallel to the edge of the board.
When making mortises, you would also need to make sure one edge of the mortise chisel was parallel to the fence.
Presuming the piece being mortise is parallel in thickness, the hooks should be set just above the top of the workpiece, maybe with the thickness of a piece of paper or two between the workpiece and the hooks.
The fence can also be used when drilling holes, to drill holes parallel to a flat edge, and if the piece being drilled is wide enough, the hooks can be used to prevent the piece being drilled from lifting on the drill bit.