Is it needed? HeckIfIKnow lol! From my days riding with Weinmann's I can't recall actually. My guess is it probably is but it will still "work" without it...but it may wobble even when tight or under hard braking.

If it were my brake I's go to a good hardware store and get a Nylon or Delrin washer to put in there. Might need to sand it down or ream it out but that can be done.