The Support Flange "supports" a large number of moving parts known as the wippen assembly and is secured to the Support Rail by screw. The three rails mentioned here and above are sometimes made of wood, sometimes extruded aluminum, but the method used here is hollow brass tubing with wooden doweling driven in the center.
The brass is considered more durable and has less tendency to add a metallic quality to the piano's tone and the wooden dowel helps to provide better stability for the screws that run through the rail as well as eliminates unwanted metallic resonance.
