The head of the adjustment screw is stripped. So an Allen wrench won't prevent it from moving with the lock nut. So, I may just mark the position of the screw and monitor it to see if it is turning.

If it is, I guess I could epoxy a 5 or 6mm bolt into the screw's head, and use that to hold it while I tighten the lock nut. As of now, however, neither the screw nor the lock nut seem to be turning.