What they did there was take a rifle and turn it into an SBR (legal), the took a Short Barreled RIFLE, and turned it into a pistol (illegal).
If they have a letter, it's worthless because no matter what they tell you it was made from a rifle. Even if you call foul and say "No it wasn't! It was made from an SBR!", an SBR is still a rifle. ATF has always said once a rifle, always a rifle (long gun).
One that always got me was, an SBR in an operational briefcase is an AOW, so is it now a 2 stamp gun, or a 1 stamp gun that is an AOW no matter if the gun is in the case or not? ATF would never give me an answer.