Licensed manufacturers and importers must legibly identify each firearm they manufacture or import, and the requirement divides into what goes where. On the frame or receiver, by engraving, casting, stamping or otherwise conspicuously placing it, goes an individual serial number, applied in a manner not susceptible of being readily obliterated, altered or removed, and not duplicating any serial number the licensee has placed on any other firearm. The frame or receiver must also carry either the licensee’s name — or recognized abbreviation — with the city and State where they maintain their place of business, or their name together with a serial number beginning with their abbreviated Federal firearms license number as a prefix, that abbreviation being the first three and last five digits, followed by a hyphen.

A second set of information may go on the frame or receiver, or on the barrel or pistol slide where applicable, and to the same durability standard: the model, if a designation has been made; the caliber or gauge; the name of the foreign manufacturer where applicable; and, for an imported firearm, the country in which it was manufactured. A part that is a frame or receiver but is not a component of a complete weapon when the licensee disposes of it — sold, shipped, or otherwise disposed of separately — must carry the serial number and all the additional identifying information too, except that the model designation and the caliber or gauge may be omitted if unknown at the time the part is identified.

The specifications are measurable rather than impressionistic, which is what makes them enforceable, and the regulation says how each measurement is taken. Print size is measured as the distance between the latitudinal ends of the character impression bottoms — the bases, not the visible outline. Depth is measured from the flat surface of the metal and not from the peaks or ridges of the impression, which is why a marking can look substantial and still fall short. The timing is fixed too: a complete weapon that is not a National Firearms Act firearm must be identified not later than the seventh day following the date the entire manufacturing process ended, or before disposition, whichever is sooner — and firearms awaiting materials, parts, or equipment repair are presumed, absent reliable evidence to the contrary, to still be in the manufacturing process.