Mike B and Kendell I,
I once worked around some metalugists and I think they had a process they called Acid Etching, I'm not sure if it is the same the police use or not. Stamping the S/N causes a difference in stress in the metal to some depth that is much more than the visual, and that process brings it out.
You have to wonder what people are thinking, ground off s/n's would stand out like a sore thumb to anyone knowing the equipment.
Dave