From your link:
Individuals manufacturing sporting-type firearms for their own use need not hold Federal Firearms Licenses (FFLs).
However, we suggest that the manufacturer at least identify the firearm with a serial number as a safeguard in the event that the firearm is lost or stolen.
Also, the firearm should be identified as required in 27 CFR 478.92 if it is sold or otherwise lawfully transferred in the future.
As I posted earlier, ATF suggest serializing, but does NOT require it.
As for selling or otherwise transferring, 27 CFR 478.92, as a LAW only applies to MANUFACTURERS and once again ATF says "should" as apposed to the verbage in the law that says
(a)(1) Firearms. You, as a licensed manufacturer or licensed importer of firearms, must legibly identify each firearm manufactured or imported as follows:
So as stated above, ATF suggest but does NOT require it.