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The hole on the deck immediately above the flame deflector through which the Technology Test Bed engine's exhaust passed. In the Saturn day's the S-IC's engines would have been located at approximately this level (although the hole in the deck would have been substantially larger).

Picture 1 of 2.

The TTB engine was located on south of center of the test stand.

One of the flame deflector walls is visible at left through the hole in the deck.