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Detail of the base of the boattail.

Plexiglas surrounds the LR-105 sustainer engine; I believe that a flexible flame curtain would have been mounted here (to allow the engine to gimbal while keeping any recirculating gases out of the boattail interior).

Note the large grey port; it would appear that this is the fuel fill/drain (there's a large duct leading from this port to the base of the fuel tank).

I believe the two spherical tanks inside the boattail are vernier start tanks.