i had wondered how the back of the frame could get blown off an auto pistol w/o seriously injuring the shooters hand.

typically the ATF considers the part that accepts the barrel to be the "gun", the part that's required to be serial numbered and tracked. auto pistols are an exception and a little odd in that regard as it's the "frame" is the part of the gun that goes in the hand. the only exception i can think of offhand to that is the ruger 22 auto which has a receiver into which the barrel is fitted and that assembly then mounting to the frame. in that case the receiver is the numbered part and the (grip)frame is not. i'm sure there are other designs that are out of the norm which likewise are outside the norm.