There Is a story about them mate.
When the shoes wear and there is no adjustment left you can't move the arm round a spline as there is a big plain bit----unless you file one spline out of the arm, but then of course the indicator points too close to the mark so it becomes what is known in the trade as a wheeee bit(place in palm of right hand and throw over shoulder whilst yelling wheeeee!) some times also know as bungmebits
