This gives the acceptable serial number definition,
http://www.atf.gov/publications/download/p/atf-p-5320-8/atf-p-5320-8-chapter-7.pdfSection 7.4 The identification of firearms
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7.4.1 Serial numbers . Each manufacturer of a firearm must legibly identify it by engraving, stamping (impressing), or otherwise conspicuously placing on the firearm’s frame or
receiver an individual serial number not duplicating any serial number placed by the manufacturer on any other firearm.
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The requirement that the marking be “conspicuously” placed on the firearm means that the marking must be wholly unobstructed from plain view. For firearms manufactured on or after
January 30, 2002, the serial number must be to a minimum depth of .003 inch and in a print size no smaller than 1/16 inch.
7.4.1.1 What is an acceptable serial number?
Alpha characters (letters), for example a name, are not acceptable as a serial number. A proper serial number may contain such characters or letters, but it must have at least one numeric character (number). ATF takes the view that marking “legibly” means using exclusively Roman letters (A, B, C, and so forth) and Arabic numerals (1, 2, 3, and so forth).
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Deviations from this requirement have been found to seriously impair ATF’s ability to trace firearms involved in crime.