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One of the signs accompanying the upper stage mockup.

It reads

Horizontal Upper Stage
PADD
Performance, Analysis & Design Demonstrator
Marshall Space Flight Center - Human Engineering Analysis Team

The Interstage is the primary structual interface between the CLV First Stage and Upper Stage. Contained inside the Interstage compartment are the Thrust Cone and J-2X Engine. When the First Stage falls away from the vehicle, this separation occurs along the seam between the Interstage and the Aft Skirt.