The old Marlin pump shotguns did not have Damascus barrels. But Marlin did indeed put out a warning not to shoot the old exposed hammer shotguns. It was Marlin's way of trying to protect themselves from legal action if some idiot fired heavy loads in their old shotguns. I figured they could have simply not endorsed shooting them, but they chose to publicly take a stand against shooting them.
The reason these guns are not strong enough for modern high base shells is the lock up mechanism's design. It simply will unlock and drive the bolt back if the load is too stout. I know a fair number of guys who collect and shoot the old Marlin shotguns, and poo-poo the "don't shoot them" statement. They say they load mild trap loads in theirs and shoot them regularly. But it takes common sense to shoot ANY old gun, and that seems to be a rare thing these days.