Any advantage to assigning my own like I do with the guns I build.
Hmmm, to get to the best answer, I suppose I would turn that question around... in two ways.
1) Is there any disadvantage to assigning my own serial number?
2) Is there any disadvantage to not doing anything about this possible problem?
My answers...
1) certainly is a disadvantage to cutting a new number in to the metal of an old firearm. Obviously, it's altering an otherwise original gun. It's simply not possible to do this without lowering the "value" of the firearm (no matter if it's worth a bunch or worth very little) and there's no possible way that it's going to look good, because it's obviously not going to match everything else stamped on the firearm. It's going to look like what it is... something added
YEARS later. And there's also the work involved in doing it.
2) I just can't see any disadvantage to doing absolutely nothing. The law states that you cannot even attempt to remove a serial number. The rest is ambiguous. A number that is fading or was never there all to clearly from the get-go is well out of your hands and there is no legal ramifications to it, and it is nobody's business in the world.
Record the number in your records/computer, take many pictures and get good pictures. Beyond that, I wouldn't do a thing and I wouldn't bring it up again. And for sure, I wouldn't contact the ATF about it. Every single thing they do is paid for by our tax dollars, they can waste that money a-plenty without our help.