Those keyed washers with a flat or a small tongue on them were just not ya good idea from the French. They just end up fouling the threads on a lot of steerer tubes, especially if the turn together with the lock nut when tightening or removing, as a lot of them fitted loosely around the steerer tubes.
I always just grind off the flats or the tongues off these washers with my Dremel to avoid such damage......
I'd do the same to centerpull brake cable hanger/stops like you have there, if it has a flat or a tongue on it.