Firearms manufactured after 1968 are required to be marked with a valid Serial Number.
If you come across one and you need to Acquire it you can do so. ATF Form 4473 Page 6, Section D states "Should you acquire a firearm that is legally not marked with a serial number (i.e. pre-1968); you may answer question 26 with "NSN" (No Serial Number), "N/A" or "None."" You cannot leave the Serial Number field blank. FastBound allows Items with NSN, N/A, and None with or without the quotes.
Although N/A, None, and NSN, are valid entries FFLGuard strongly recommends against using them on any part of ATF Form 4473 or the A&D Record. The Serial Number section is always applicable as it is required information. In instances where there is no Serial Number visible on the receiver or frame, FFLGuard recommends putting "None Marked".
You should be diligent to be certain that the firearm is pre 1968 and does not have an obliterated Serial Number or a Serial Number in an unconventional location. It is up to you to ensure that the Serial Number never existed as opposed to having been removed, obliterated, or concealed or in an unusual place. If it has a Serial Number, the FFL is responsible to find it. If the Serial Number was removed or obliterated, the firearm is contraband and cannot be possessed. This analysis should be done upon receipt of the firearm and the FFL is responsible to be certain that any firearm it receives into inventory is logged properly and not accepted if it is contraband.
If the firearm's Serial Number has been removed or obliterated do not take possession of it and if you already have then Acquire it into FastBound and immediately contact your local law enforcement.
If the firearm was a privately completed (i.e. 80% lower), the firearm must be identified as required in 27 CFR 478.92 if it is sold or otherwise lawfully transferred.
For more complete information you can contact your compliance attorney or FFLGuard if you are a client of theirs.