model 12 ducks unlimited |Winchester Shotguns|Forum|Winchester Collector

Hi hoping someone can help me, I’m looking at buying a model 12 heavy barrel ducks unlimited. Where the tube mag clamps onto the barrel their is a slot cut into the barrel right where it clamps onto the barrel hard to describe but the gun smith has told me this is normal I can’t see having a heavy barrel then notching it halfway down the barrel. It does not go through just kinda notched. Any help is appreciated. Thanks norm.