I wouldn't have known it was welded if you didn't say anything. Who ever did it did a great job!
Ren is right, technically it was not reassembled correctly, and if the man were to see it, it would likely bring some trouble with it. If it were mine, I'd take the parts off of the receiver for another build (like a JRA GI Heel receiver), and hide- I mean get rid of the receiver.
Funny thing (or not) is that those types of welded receivers show up in pawn shops and what once in a while. Most people are under the assumption that if you cut the lug off, you're good to go (which is how it should be, or to have a selector lock welded in place like the military match rifles), but I digress.
Hopefully with the new el presidente, some of the gun laws will start to be relaxed, and some of the regs like "once a machinegun" could be one of them.