Two situations complicate the ordinary marking rule, and neither is obvious from the general requirement. The first is a frame or receiver made in more than one piece. The subpart to be identified is the modular subpart that is the OUTERMOST housing or structure designed to house, hold, or contain the primary component — the sear or equivalent in a handgun, the breech blocking or sealing component in a projectile weapon other than a handgun, or the internal sound reduction component of a muffler or silencer. Where more than one subpart is similarly designed to hold that component, left and right halves being the regulation’s own example, EACH must carry the same serial number and associated licensee information, not duplicated on any other frame or receiver.

A restriction follows from that. The identified subpart or subparts of a complete multi-piece frame or receiver — assembled or unassembled — shall not be removed and replaced, and the regulation points to § 478.34 and to the statutes on obliterated markings for why. Three conditions must ALL hold for a replacement to be permitted: the replacement subpart is not itself a firearm under 26 U.S.C. 5845; it is identified by the licensed manufacturer of the original subpart with the same serial number and associated licensee information in the manner this section prescribes; and the original subpart is destroyed under that manufacturer’s control or direct supervision before the replacement is placed. The practical reading is that the identified subpart is not a consumable part: swapping it is a marking question before it is a repair question, and the old one has to be destroyed rather than merely set aside.

The second situation runs the other way and saves work rather than creating it. Licensees may ADOPT existing markings previously placed on a firearm rather than marking it afresh. A licensed manufacturer may adopt the serial number and other identifying markings placed by another licensed manufacturer, provided the firearm has not been sold, shipped or otherwise disposed of to anyone other than a licensee, and the adopted serial number is not duplicated on any other firearm. Licensed manufacturers and importers may also adopt markings on a remanufactured or imported firearm that already meets this section’s requirements and HAS passed to a non-licensee, subject to the period and manner the regulation prescribes. Separately, where engraving, casting or stamping a weapon would be dangerous or impracticable, the Director may authorize other means of identification on a letter application showing that and showing the alternative will not hinder administration of this part.