the hozan BB lockring tool has a smaller spanner on one side for this purpose. headsets like these are very common on 80s japanese bikes, although i don't know who made them, tange or hatta i guess. my theory on the design is that you can fudge with brake cable stops or reflector brackets without readjusting the headset. nice in theory, but can be a real pain to adjust sometimes.

i've never seen one of these with a set screw.