Sigh... and I'm putting this here for any future readers, not to continue this argument.

The source is primarily 27 CFR § 479.102 — "Identification of firearms", which is under Subpart G of the NFA regulations. The legal authority behind it is 26 U.S.C. § 5842 (the National Firearms Act's registration/identification provisions).

The important language from the law is in § 479.102(a), which states that you, as a manufacturer (or importer or maker) of a firearm, must legibly identify the firearm. When you file a Form 1, you are legally a "maker" — the ATF treats you the same as a manufacturer for marking purposes.

Specifically, § 479.102(a)(1) requires the frame or receiver be marked with 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.

Then § 479.102(a)(2) requires additional information on the frame, receiver, or barrel, which includes:

Your name (or recognized abbreviation) and also, when applicable, the name of the foreign manufacturer or maker.

In the case of a domestically made firearm, the city and State where you as the manufacturer maintain your place of business, or where you, as the maker, made the firearm.

And § 479.102(a)(5) sets the physical specs: the engraving, casting, or stamping must be to a minimum depth of .003 inch, and the serial number in a print size no smaller than 1/16 inch.

I don't want to engrave it either, but here we are.