Text below copied from the /r/gunbuilds post on reddit :

It appears the ATF is doing to P80 what they did to Ares 6'ish years ago. This shouldn't affect builders that have already purchased parts but the seizure of records always means theres a possibility previous, current and prospective customers may be on lists now possessed by the ATF.

There is no federal requirement to serialize self manufactured firearms but if you feel obligated so serialize your personally built firearms for your own records or premption of state laws, ATF serial number requirements here or more specifically 2002 in section 27, part 478.92 of the Code of Federal Regulations.

Note that depth requirements are based on thickness from flat surfaces of metal. At a minimum, the Serial number itself should be engraved on a metal part insert (ie. glock) though it is often easier to make all identifying marks on a metal insert (ie. HKI)