And where would one go to determine ones SN? You don’t “register” so essentially you’re engraving your own random number into a lower, means nothing, never traveled trough an FFL so it just engraving in metal wouldn’t help or hurt their cause. Negligible
If we were talking a form 1 it will have engraved credentials from you when you manufactured it and a serial number which was either registered during the NFA process or originally came with the weapon you form 1d
The one I made for my Nephew I did a form 2 with a made up serial number(his birthday) and my info as the manufacturer. but Im an 07 FFL