Roland,
Stamping into the steel of a gun changes the steel immediately around the stamp--compresses it and "swells" it a bit. If the stamp is made before the pistol is blued this swelling is usually smoothed away by final polishing.
If the stamp is made after the pistol is blued the swelling is not removed and the compression alters the nature of the bluing, making it a bit lighter than the blued steel around it--this is sometimes referred to as "halo".
--Dwight
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