Some time ago I saw what looked like a NEF .410 at a Gander Mountain. The barrel was marked Marlin Model 200, I bought it thinking that it would be the same receiver and I might buy a 28 or 20 GA barrel for it. Called NEF and to find out more about it and was told they were a special run for Kmart stores and they would not fit additional barrels to that frame. What makes it different from a regular SB1 frame?