"6.2.1 ...
All NFA firearms must be identified by a serial number and other specified markings. If an existing
firearm is being used in the making of the NFA weapon, and that firearm is serialized, the existing serial
number should be used (unless it duplicates a serial number already used by the maker on Form 1) and
entered in Block 4(g). If the weapon is of new manufacture, the applicant must assign a unique serial
number and enter it in Block 4(g). For example, a unique serial number could be composed of at least 4
digits preceded by the initials of the maker. NOTE: alpha characters, e.g., a name, will not be accepted
as a serial number. If a name is to be used, there must be at least one numeric character in addition to
the alpha characters.
The serial number must be engraved or stamped on the receiver of the firearm and the caliber, model,
and identification of the maker must be engraved on the barrel or frame or receiver of the weapon.96 The
marking and identification requirements for a maker are the same as for a manufacturer. Refer to
section 7.4 for a detailed discussion of the requirements.