Generally, manufacturers can use whatever serial numbers they want, provided they are not duplicated and the firearms are not being manufactured for others under a variance. Serial numbers must contain either a combination of Roman letters and Arabic numerals, or solely Arabic numerals, and may contain a hyphen. ATF has confirmed that spaces need not be recorded in the A&D Record.
If a firearm is marked with a serial number containing Cyrillic or other non-Roman letters or non-Arabic numbers, it must be stamped with a compliant serial number when it is imported into the United States. If you cannot find a confirming serial number, you should send the firearm back.
Sometimes you will need to log into the A&D Record a pre-1968 firearm that does not have a serial number. In these cases, you should enter "None Marked" in the A&D. Be diligent to be certain that the firearm does not have an obliterated serial number or simply a serial number in an unconventional location. Possession of firearms with an obliterated or removed serial number is a crime.
The serial number (and other firearms description information) entries must be done with precision as errors here will follow them down the line, potentially leading to serious violations that cannot be corrected if the firearm has left inventory. The only way to be absolutely sure that an entry is correct is to examine it directly from the firearm itself.
For more information consult the FFLGuard Help Desk