remington:  As you mention, the U.S. Krag action has one locking-lug.  If that should fail, the rib on the bolt body and the bolt-handle root make contact with the receiver and serve as secondary or 'safety' lugs and help to keep the action from totally failing.  On U.S. receivers the rib and bolt handle normally do not make contact with the receiver.  On Norwegian and some custom U.S. Sporters, the bolt is fitted so that the rib on the bolt-body makes contact at its rear with the receiver.