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