Sorry, I should have clarified my post a little better but mohavesam did ask about the s/n part, which is considered to be the firearm be it a stripped receiver or a complete firearm.
Here is a partial part of ATF Ruling 20-10 that talks about this:
A dealer is "engaged in the business" of gunsmithing, as defined in 18 U.S.C. 921(a)(21)(D) and 27 CFR
478.11, when he/she receives firearms (frames, receivers, or otherwise) provided by a customer for the
purpose of repairing, modifying, embellishing, refurbishing, or installing parts in or on those
firearms. Once the work is completed, the gunsmith returns the firearms, and charges the customer for
labor and parts. As with an individual customer, a licensed dealer-gunsmith may receive firearms
(properly identified with a serial number and other information required by 27 CFR 478.92) and conduct
gunsmithing services for a customer who is a licensed importer or manufacturer. A dealer-gunsmith is
not "engaged in the business" of manufacturing firearms because the firearms being produced are not
owned by the dealer-gunsmith, and he/she does not sell or distribute the firearms manufactured. Once the
work is completed, the dealer-gunsmith returns the firearms to the importer or manufacturer upon
completion of the manufacturing processes, and does not sell or distribute them to any person outside the
manufacturing process. Under these circumstances, the licensed dealer-gunsmith is not "engaged in the
business" of manufacturing firearms requiring a manufacturer's license.
In contrast, a dealer-gunsmith may make or acquire his/her own firearms, and repair, modify, embellish,
refurbish, or install parts in or on those firearms. If the dealer-gunsmith then sells or distributes those
firearms for livelihood and profit, the dealer-gunsmith is engaged in his/her own business of
manufacturing firearms. A person engaged in the business of manufacturing firearms for sale or
distribution is required to be licensed as a manufacturer, identify/mark all firearms manufactured,
maintain permanent records of manufacture, submit annual manufacturing reports, and pay any taxes
imposed on firearm manufacturers. A licensed dealer-gunsmith who becomes licensed as a manufacturer
must also segregate all firearms manufactured for that business separately from firearms for which
gunsmithing services are being performed.