This is where the rudder will mount . Yet again all hinge locations are predrilled, except for the lowest on the vertical stab itself ( rudder has all holes pre-done ) because of inaccessibility during manufacture … no big deal .. just a hole to drill, and its ready. By the way, all gaps in control surfaces are sealed by either carbon or G-10 seal strips .. as with the ailerons, the rudder has a pre drilled and countersunk hole for the controll horn rod on the surface.