It seems to me that no matter what you do to mark it as identifiable to only you, you defeat the purpose of having a weapon that's
not identifiable to you. On the other hand, if you have to use it and it gets seized in the subsequent investigation, the cops who seize it are bound by law to document who they seized it from. Assuming you did nothing wrong in using it, you would still be the only person of record to have ever been in possession of the weapon, and the person of record from whom it was seized, so how are they going to justify not giving it back to you after you are ruled justified in shooting whomever you used it on?
Surely there are already identifying marks on some part of the weapon that only you know about though. Some minor machining marks on the receiver, or a scratch on the barrel nut from using channel locks instead of a spanner wrench to tighten it (I'm guilty of that myself), or wrench marks where you tighten the flash suppressor onto the barrel? If you were so perfect in your assembly that none of those kinds of things happened, just pull a scribe out of your toolbox and make a mark or three inside the mag-well, take a picture or three, put 'em in your safe and forget about it unless and until the weapon is either seized by cops or stolen by scumbags (sorry if I'm repeating myself - LOL). Make sure there's a time/date-stamp on any pictures you take, and make sure also that the function to save EXIF data within the digital file is turned on on the camera you use to take the pics, and keep a data-disc or flash drive containing the digital file with the pics in your safe. The visible time/date-stamp on the hard copy will correspond with the EXIF data time/date-stamp, and when combined with the identifying marks that the pictures make a visible comparison possible with, digital file and the cops' own eyeballs looking at the seized weapon, it would be pretty hard for anyone to legitimately deny that you have established ownership. I would never put it past cops to try to deny it, but however far you have to take it, you will eventually win, or at least you should.
Identifiable marks are way easier to put on your weapon and document for prosperity than building the thing was. Alternatively, you can always put a serial number on it, take it to your local FFL and have it "transferred" to you "legally." You've managed to build a working weapon though, it was perfectly legal to do, so I'm not a fan of that alternative, but then, I'm not concerned about being able to establish ownership of my weapons (if I have any to begin with -
), and you are, so there ya go.
Blues