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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).

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

Picture 1 of 2.

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