I'd use the biggest smooth file I could get. Put a bit of duct tape on the file, and you can use the pin at the far end as a guide to keep it in line with the length of the scale. The pin lightly rubbing on the file won't go through the tape, but a nude file rubbing on a taped pin-head will.

If you want to use a hand-grinder accessory, my choice would be the flat of one of the thin cutting-wheels, which you use on a screw mandrel. Be sure to wear glasses of some kind for this job, but it shouldn't break if the pressure is light.
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But it doesn't take much work with anything powered to overheat the pin. This is no big deal in some materials, but in celluloid it can create a little raised rim around the washer.