The rough side of the two keyed washers press against the stop dial which has the actual movable stop that butts up against the fixed stop on the headstock casting. That rough surface provides the resistance to further movement.
There is a limit as to how much force it will withstand without slipping.
When it slips I am sure the rough side scrapes some material from the dial.
Normal(reasonable) drilling force is not sufficient to make it slip.
i.e. not intended for a gorilla operator.
Also the amount of tightening torque applied to the stop wingnut affects the slip threshold.