Stamping a radom ser# on a firearm that left the orig mfg'rs factory w/o one, and then recording it as such in your FFL Log is not doing yourself a favor.
There's the Manufacturers applied ser#,,that which is required to be listed in the 'Book' IF it so appears on the firearm.
There is the ser# that the new law(s) now require of home-made firearms ,,those not mfg'd by FFL licensed Mfg'rs (Type 07 FFL), but made by non-licensees for their own use.
The gun you make for your own use as a non-FFL is not supposed to be re-sold or transfered. But they do make it into the trade buy/sell stream anyway.
Put your own Identifying Ser# on it and be done with it.
You cannot however act as an agent of a Firearms Mfg'r and stamp any old ser# on one of their guns that does not/did not ever have one, and then claim that the # is now the legal Ser# of that firearm.
There is an 'Identifying Characteristic' such as a random number, symbol,stamped or engraved name /feature etc, that can be placed on the firearm for ID purposes.
That Characteristic can be noted in the FFL book. There is a section on the NCIC computer entry for stolen firearm for such info if the gun gets stolen. Helps to sort out and further identify a particular piece among others that are common.
But it's not the Mfg'rs applied ser#.
Those mfg'd at a time when a ser# for that particular catagory of gun was not required and not stamped upon it are just listed as NSN in most FFL logs.
Evidence of having a ser# and it being removed are a different matter.