9.3 Mauser's answer is correct. Note the qualifier " if it's properly fitted". This applies to all third fasteners, including Greener's cross bolt, various extended top rib fasteners and others. It has been proven in more than a century of hard use that these third fasteners are not necessary on a well fitted gun, even Purdey hasn't used their third fastener for many years, finding it unnecessary on their products. In the 1860's, 70's, 80's and 1890's when metallurgy was more basic and some gun designs did not stand up to heavy use there was great preoccupation with the strength and security of these new-fangled break open guns and makers competed to offer the most robust actions. To this day these third fasteners are more of a selling point than a structural advantage and in all but the most carefully made guns ( usually meaning expensive) are not well enough fitted to offer any added strength.