This cross section view of the big boy body shows how the scriber/pencil can be held above or below the pivot point and how the brass dowel stops the knob thread from being damaged.
A threaded dowel is used on the big boy so the forces applied clamping down on the metal scribe don’t damage the thread.
Note this image shows the brass dowel in position clamping down on the metal scribe with the scribe below the pivot bolt.