Did you do a search?

We've had threads on this, with links to ATF letters and other documenting information.

Watching... I did a few searches with no luck but would love to see the link if it exists

Here are a couple of official links:

How must licensed manufacturers and licensed importers identify firearms, armor piercing ammunition, and large capacity ammunition feeding devices?
https://www.law.cornell.edu/cfr/text/27/478.92

How must firearms be identified?
https://www.law.cornell.edu/cfr/text/27/479.102

Since they seem to be slightly contradictory, the question has been asked of ATF for clarification.
I'll post what I can find in a few minutes.

that is the big thing people say you don't need to serialize it yet its all hearsay while the atf clearly states maker.

if you make a 80% into a firearm you have become the maker of a firearm.

I think people are confusing license with serial.

§479.1 General.

This part contains the procedural and substantive requirements relative to the importation, manufacture, making, exportation, identification and registration of, and the dealing in, machine guns, destructive devices and certain other firearms under the provisions of the National Firearms Act (26 U.S.C. Chapter 53).

that section is not only about nfa , it falls under "AND CERTAIN OTHER FIREARMS"

which under 27 cfr is pretty much anything other than explosive or machine gun

Firearm. (a) A shotgun having a barrel or barrels of less than 18 inches in length; (b) a weapon made from a shotgun if such weapon as modified has an overall length of less than 26 inches or a barrel or barrels of less than 18 inches in length; (c) a rifle having a barrel or barrels of less than 16 inches in length; (d) a weapon made from a rifle if such weapon as modified has an overall length of less than 26 inches or a barrel or barrels of less than 16 inches in length; (e) any other weapon, as defined in this subpart; (f) a machine gun; (g) a muffler or a silencer for any firearm whether or not such firearm is included within this definition; and (h) a destructive device. The term shall not include an antique firearm or any device (other than a machine gun or destructive device) which, although designed as a weapon, the Director finds by reason of the date of its manufacture, value, design, and other characteristics is primarily a collector's item and is not likely to be used as a weapon. For purposes of this definition, the length of the barrel having an integral chamber(s) on a shotgun or rifle shall be determined by measuring the distance between the muzzle and the face of the bolt, breech, or breech block when closed and when the shotgun or rifle is cocked. The overall length of a weapon made from a shotgun or rifle is the distance between the extreme ends of the weapon measured along a line parallel to the center line of the bore.

Q: How must licensed manufacturers and licensed importers identify firearms, armor piercing ammunition?