Following the white topcoat, fuel sight gauges were installed on the inboard side of the root skins (viewable from the cockpit). After the fuel fittings for the sight gauges were installed, the forward and middle root skins were riveted to the root rib.
Grommets were installed for fuel fitting, pitot and flap cable penetrations. The pitot tubes and flap cable run through a third aft root skin piece that is screwed to the root rib for easy removal to facilitate inspection/maintenance.
Here's a closeup of the fuel sight gauge.
