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Forensics techs would clean and smooth the surface where the numbers were stamped then apply an acidic solution there. Where the stamping action takes place it condenses the grain of the steel in similar fashion to forging processes. The tighter bond grain of the metal will show in a different gray scale shade than the non-stamped metal. I don't know which acid solution they use or how much dilution but that is the principal way they do it on wepons and other objects that have been altered to change or remove identifying serial numbers. If the area is deeply pitted with corrosion, it may not have anything left to bring back up.
Kerby