This Volquartsen Bolt Stop Pin and V-Lock Retainer Spring is a quality replacement for the factory parts in your Ruger MK IV pistol. These parts will replace the factory bolt stop pin and bolt stop cross pin retainer in the Ruger MK IV frame. The Bolt stop Pin and V-Lock Retainer Spring will increase durability as well as prevent the cross pin from backing out of the frame during extended periods of shooting.