I once worked for a company that did quite the same thing with AR-10 receivers. We bought them in Portugal, shipped them to Canada and de-milled them per Canadian law which was saw cut into two pieces. From that point on, they were "parts" and could be shipped into the US. Once in the US, we re-welded the receivers together and we had AR-10's that actually said Armalite AR-10 on them. We made about 200 of those and I've never seen one since.
Anyhow, once welded back together that becomes a newly manufactured receiver and must contain the manufacturers information stamped, etched on the receiver. So that's your assurance of legality, a licensed manufacturer did indeed make the rifle, and there is a LOT of documentation showing it really happened and happened legally at one point. I want to say I read about it in one of the M14 books and there may even be a book with a photo of the letter.
It's a good idea to get a letter on a Hahn rifle as the re-weld thing has been contentious throughout time. I know companies that been given letters only to have ATF change their minds. I know guns have been confiscated at times. If confiscated (rightfully or wrongfully) you're quite unlikely to ever get the rifle back. The letter could save the confiscation. Best of luck to you sir. Get on the M14 forums and network around, someone will have a copy of that letter. I'm really surprised that it's not easily found on the internet. It would be in everyone's best interest to have a copy of that letter plastered EVERYWHERE.