Those are Left and right adjustments, there should be bolts through the outer two holes with a nut on each side of the plate. This allows you to adjust left/right and tighten it so it does not "flop around" on your hitch.
Here is a photo of the slot hitch adapter. (foreground).
And in the back you can see a single hole "drop sleeve" adapter for
the brinley plow. You likely have the 3-hole drop sleeve adapter (wider than one shown)
The "slot hitch adapter" fits into the drop sleeve adapter and is adjusted
with the bolts shown horizontal in the plate.