[“At one point in time, it may have been [registered], but it’s going to be impossible to find out.”]
The FBI lab can usually raise these numbers if they are stamped on.
Yep. If you've ever seen how footprints of compressed snow will sometimes remain after the surrounding snow has melted, it's an analogous process. When a serial # or other text is stamped into a metal it compresses the metal under the stamp increasing its molecular density as compared to surrounding metal. Even if the serial number is filed or worn down to the bottom of the impressions (i.e. the stamped digits and/or letters), the metal underneath will be denser, and when given a modulated acid bath to remove the less dense surrounding metal, they can generally be raised.