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Overall view up the second stage adapter, taken from approximately Pos. III (LOX tank 3 and fuel tank 3 are at center-left on the bottom; note the 7-inch LOX vent and the fuel vent on these tanks' forward skirts).

Note the the ends of the spider beam between the propellant tanks. If you look closely, you can see the lines connecting the triplex spheres (containing nitrogen to pressurize the fuel tanks) pass through holes in the spider beam.

The screen installed over the opening between the S-I stage and its adapter is presumably to keep the birds out; I don't believe that it was original equipment.