The spacer washer isn't there all the time. I suspect it's included only when the rotor is found to touch the bridges when it spins. Since the rotor bearing block is press fitted into the rotor, there will be a certain amount of variability between different rotors in terms of where the rotor sits relative to the rotation plane of the ball bearings. Rather than tightening the tolerance for the height of the bearing block within the rotor (as I assume gen Rolex rotors are made), I assume that both Shanghai and Dandong simply have an optional spacer washer that can be installed during assembly to ensure that the rotor clears the bridges by a certain distance.
