I suppose there are other possibilities, but the most common method is with a "lacquer stick", a white waxy crayon-type marker that you rub on, then wipe off the excess, leaving the white in the channels of the stamped or engraved numbers/letters/markings. One notable exception is the "Giesert" marking under the safety, that came from the factory filled with white paint. None of the other markings were originally "whited" in.
I still have part of one lacquer stick that I bought form Ralph Shattuck about a gazillion years ago.
dju
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