Not a Thompson related comment but an ATF comment from an experience I had a number of years ago. I filed a form 4 transfer for a MP5 (and so marked) made into an SBR from a HK94. When the approved stamp came back the ATF reviewer crossed our MP5 and wrote in MP5K for some reason. Of course the two are different and have different barrel lengths. I had to send in photos of the actual gun to prove it was really an MP5. Of course I also sent in photos of a MP5K along with an explanation of the difference. Strange...