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Overall view of the inside of the APS engine thrust chamber. Compare with the hot-fired section on display at the Cradle of Aviation.

Note the darker-colored ablator at the center of the thrust structure. The color of the rest of the thrust chamber is a bit washed out in this photo; it is slightly more clay-colored.

This is the second APS thrust chamber (i.e., the one which was inaccessible when I took the previous batch of photos); once tipped over, the light available in the interior of the engine was more favorable than the other.

The darker lines around the periphery of the aft end of the thrust structure and the darker line going down the side at about the 8:00 position appear to be water stains, rather than any sort of structural damage. I understand that the thrust chambers were acquired by the former Michigan Space & Science Center from a federal surplus action at Johnson Space Center, where they'd been located in a back storage area, so it's possible that the discoloration happened long ago.