ipsilateral_7 is correct, you can't just convert a MG back and forth as you please... once a MG, always a MG: up until the point it is destroyed per BATFE specs....welding up the safety sear hole is
NOT "per spec". But torch cut in the approved places is... If you welded the hole, that implies that you possessed the MG before you neutered it. Prosecutable to10 years and $250K baby. Metalurgical exam would easily reveal it. Burden of proof might be on the alphabet guys, but WHY the heck would you bump heads with that one!
Now the real argument would be to say that re-welding a torch cut MG receiver (with the hole welded up too) would be considered a MG or not.... but I don't think anyone in good conscience would try to do that either, but it is arguable; unless I am missing something in the regs...Regardless, now you know why none of the parts kits suppliers aren't giving you much if any of the receiver parts anymore... there is no law against owening them as long as it was demilled properly, but these distibutors sure as hell don't want to deal with their customers experimenting with that "grey line", and having it come back to bite them in the butt.... Some importers are instructed at the bonded wharehouses to not include receiver parts either... which I think is B.s., but whatchya gonna do..... ya want the rest of the parts or not?
Point being, stay the heck away from the bloody third hole, toss out your safety sears (not that there is a law, statute or ruling that says this.... that I know of) out and have fun.
-C