Knobs to fit on the M4 nuts underneath the heated bed on a Longboat prusa.

These are longer than some I've seen as they have to miss the bearing holders on the Y carriage. The size may need adjusting or some shims used depending on the accuracy of your prints.
Note this bed is altered slightly from the original design in that it now has a glass top and springs between bottom and top plates.

Video of adjusters in action.
http://youtu.be/sXvYc09dssc