Ken, That would be true if the mag locking lugs went into the mag well but they rest against the mag catch outside of the mag well.. The way I read it Riesch was saying the grooves were part of the way the T.H. was made and some manufactures didn't need to make them, thus some T.H. have the smooth mag well. I would think that eliminating those two slots would have really sped up production and it would have been across the board.