§ 479.102 Identification of firearms.

(a) Identification required.  Except as otherwise provided in this section, you, as a manufacturer, importer, or maker of a firearm, must legibly identify the firearm as follows:

(1) Serial number, name, place of business.  By engraving, casting, stamping (impressing), or otherwise conspicuously placing or causing to be engraved, cast, stamped (impressed) or otherwise placed on the frame or receiver thereof, an individual serial number, in a manner not susceptible of being readily obliterated, altered, or removed. The serial number must not duplicate any serial number placed by you on any other firearm. The frame or receiver must also be marked with either: Your name (or recognized abbreviation), and city and State (or recognized abbreviation) where you as a manufacturer or importer maintain your place of business, or in the case of a maker, where you made the firearm; or if a manufacturer or importer, your name (or recognized abbreviation) and the serial number that begins with your abbreviated Federal firearms license number, which is the first three and last five digits, as a prefix to a unique identification number, followed by a hyphen, e.g., “12345678-[unique identification number]”; and

(2) Model, caliber or gauge, foreign manufacturer, country of manufacture.  By engraving, casting, stamping (impressing), or otherwise conspicuously placing or causing to be engraved, cast, stamped (impressed) or placed on the frame or receiver, or barrel or pistol slide (if applicable) thereof certain additional information. This information must be placed in a manner not susceptible of being readily obliterated, altered, or removed. The additional information shall include:

(i) The model, if such designation has been made;

(ii) The caliber or gauge;

(iii) When applicable, the name of the foreign manufacturer or maker; and

(iv) In the case of an imported firearm, the name of the country in which it was manufactured. For additional requirements relating to imported firearms, see Customs regulations at 19 CFR part 134.

(3) Multi-piece frame or receiver.  In the case of a multi-piece frame or receiver, the modular subpart that is the outermost housing or structure designed to house, hold, or contain either the primary energized component of a handgun, breech blocking or sealing component of a projectile weapon other than a handgun, or internal sound reduction component of a firearm muffler or firearm silencer, as the case may be, shall be the subpart of a multi-piece frame or receiver identified in accordance with this section. If more than one subpart is similarly designed to house, hold, or contain such primary component (e.g., left and right halves), each of those subparts must be identified with the same serial number and associated licensee information not duplicated on any other frame or receiver. The identified subpart(s) of a complete (assembled or unassembled) multi-piece frame or receiver shall not be removed and replaced (see § 478.34 of this subchapter, 18 U.S.C. 922(k), and 26 U.S.C. 5861(g) and (h)), unless—

(i) The subpart replacement is not a firearm under 26 U.S.C. 5845;

(ii) The subpart replacement is identified by the qualified manufacturer of the original subpart with the same serial number and associated licensee information in the manner prescribed by this section; and

(iii) The original subpart is destroyed under the manufacturer's control or direct supervision prior to such placement.

(4) Frame or receiver, machine gun conversion part, or silencer part disposed of separately.  Each part defined as a frame or receiver or modular subpart thereof described in paragraph (a)(3) of this section, machinegun, or firearm muffler or firearm silencer that is not a component part of a complete weapon or complete muffler or silencer device at the time it is sold, shipped, or otherwise disposed of by you must be identified as required by this section with an individual serial number not duplicated on any other firearm and all additional identifying information, except that the model designation and caliber or gauge may be omitted if that information is unknown at the time the part is identified.

(5) Size and depth of markings. The engraving, casting, or stamping (impressing) of the serial number and additional information must be to a minimum depth of .003 inch, and the serial number and any associated license number in a print size no smaller than 1⁄16 inch. The size of the serial and license number is measured as the distance between the latitudinal ends of the character impression bottoms (bases). The depth of all markings required by this section is measured from the flat surface of the metal and not the peaks or ridges.

(6) Period of time to identify firearms.  You, as a manufacturer, shall identify a complete weapon or complete muffler or silencer device no later than close of the next business day following the date the entire manufacturing process has ended for the weapon or device, or prior to disposition, whichever is sooner. You must identify each part or modular subpart defined as a machine gun (frame or receiver, or conversion part) or muffler or silencer that is not a component part of a complete weapon or complete muffler or silencer device at the time it is sold, shipped, or otherwise disposed of no later than close of the next business day following the date the entire manufacturing process has ended for the part, or prior to disposition, whichever is sooner. For purposes of this paragraph, firearms awaiting materials, parts, or equipment repair to be completed are presumed, absent reliable evidence to the contrary, to be in the manufacturing process. Importers must identify imported firearms within the period prescribed in § 478.112 of this subchapter.

(7) Meaning of marking terms.  For purposes of this section, the term “identify” means placing marks of identification, the terms “legible” and “legibly” mean that the identification markings (including any unique identification number) use exclusively Roman letters (e.g., A, a, B, b, C, c) and Arabic numerals (e.g., 1, 2, 3), or solely Arabic numerals, and may include a hyphen, and the terms “conspicuous” and “conspicuously” mean that the identification markings are capable of being easily seen with the naked eye during normal handling of the firearm and are unobstructed by other markings when the complete weapon or device is assembled.

(b) Exceptions —

(1) Alternate means of identification.  The Director may authorize other means of identification to identify firearms upon receipt of a letter application or prescribed form from you showing that such other identification is reasonable and will not hinder the effective administration of this part.

(2) Destructive devices.  In the case of a destructive device, the Director may authorize other means of identification to identify that weapon upon receipt of a letter application or prescribed form from you. The application shall show that engraving, casting, or stamping (impressing) such a weapon as required by this section would be dangerous or impracticable and that the alternate means of identification proposed will not hinder the effective administration of this part.

(3) Adoption of identifying markings. You may adopt existing markings and are not required to mark a serial number or other identifying markings previously placed on a firearm in accordance with this section, as follows:

(i) Newly manufactured firearms.  Manufacturers may adopt the serial number and other identifying markings previously placed on a firearm by another manufacturer provided the firearm has not been sold, shipped, or otherwise disposed of to a person other than a qualified manufacturer, importer, or dealer, and the serial number adopted is not duplicated on any other firearm.

(ii) Remanufactured or imported firearms.  Manufacturers and importers may adopt the serial number or other identifying markings previously placed on a firearm that otherwise meets the requirements of this section that has been sold, shipped, or otherwise disposed of to a person other than a licensee provided that, within the period and in the manner herein prescribed, the manufacturer or importer legibly and conspicuously places, or causes to be placed, on the frame or receiver either: Their name (or recognized abbreviation), and city and State (or recognized abbreviation) where they maintain their place of business; or their name (or recognized abbreviation) and abbreviated Federal firearms license number, which is the first three and last five digits, individually (i.e., not as a prefix to the serial number adopted) after the letters “FFL”, in the following format: “FFL12345678”. The serial number adopted must not duplicate any serial number adopted or placed on any other firearm, except that if an importer receives two or more firearms with the same foreign manufacturer's serial number, the importer may adopt the serial number by adding letters or numbers to that serial number, and may include a hyphen.

(iii) Manufacturers performing gunsmithing services.  Manufacturers may adopt the serial number or other identifying markings previously placed on a firearm by a qualified manufacturer, importer, or dealer, provided the manufacturer is performing services as a gunsmith (as defined in § 478.11 of this subchapter) on existing firearms not for sale or distribution.

(4)

(i) Firearm muffler or silencer parts transferred between qualified manufacturers for further manufacture or to complete new devices.  Manufacturers qualified under this part may transfer a part defined as a muffler or silencer to another qualified manufacturer without immediately identifying or registering such part provided that it is for further manufacture (i.e., machining, coating, etc.) or manufacturing a complete muffler or silencer device. Once the new device with such part is completed, the manufacturer who completes the device shall identify and register it in the manner and within the period specified in this part for a complete muffler or silencer device.

(ii) Firearm muffler or silencer replacement parts transferred to qualified manufacturers or dealers to repair existing devices.  Manufacturers qualified under this part may transfer a replacement part defined as a muffler or silencer other than a frame or receiver to a qualified manufacturer or dealer without identifying or registering such part provided that it is for repairing a complete muffler or silencer device that was previously identified and registered in accordance with this part and part 478.

(5) Frames or receivers designed before August 24, 2022.  Manufacturers and importers may continue to identify the same component of a firearm defined as a frame or receiver as it existed before August 24, 2022 with the same information required to be marked by paragraphs (a)(1) and (a)(2) of this section that were in effect prior to that date, and any rules necessary to ensure such identification shall remain effective for that purpose. Any frame or receiver with a new design manufactured after August 24, 2022 must be marked with the identifying information and within the period prescribed by this section. For purposes of this paragraph, the term “new design” means that the design of the existing frame or receiver has been functionally modified or altered, as distinguished from performing a cosmetic process that adds to or changes the decoration of the frame or receiver (e.g., painting or engraving), or by adding or replacing stocks, barrels, or accessories to the frame or receiver.

(c) Voluntary classification of firearms.  The Director may issue a determination (classification) to a person whether an item, including a kit, is a firearm as defined in this part upon receipt of a written request or form prescribed by the Director. Each such voluntary request or form submitted shall be executed under the penalties of perjury with a complete and accurate description of the item or kit, the name and address of the manufacturer or importer thereof, and a sample of such item or kit for examination. A firearm sample must include all accessories and attachments relevant to such classification as each classification is limited to the firearm in the configuration submitted. Each request for classification of a partially complete, disassembled, or nonfunctional item or kit must contain any associated templates, jigs, molds, equipment, or tools that are made available by the seller or distributor of the item or kit to the purchaser or recipient of the item or kit, and any instructions, guides, or marketing materials if they will be made available by the seller or distributor with the item or kit. Upon completion of the examination, the Director may return the sample to the person who made the request unless a determination is made that return of the sample would be or place the person in violation of law. Except for the classification of a specific component as the frame or receiver of a particular weapon, a determination made by the Director under this paragraph shall not be deemed by any person to be applicable to or authoritative with respect to any other sample, design, model, or configuration.

[ATF-2021R-05F, 87 FR 24747, Apr. 26, 2022; 87 FR 51250, Aug. 22, 2022]

So in short, the lower does not have to be engraved which is what I was hoping to be the case. It wouldn't make sense to have to serialize a non-serialized part. I know the ATF wants us to, but I'm not doing that if the law doesn't obligate me to.

They said "they want the lower to be marked"
Not some like "legally it has to like the upper"

They can "want" anything but unless legally required to, in personally wouldn't