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Close-up of the transporter, actuator arm, and thrust structure.

This S-IC, as well as the one at KSC and the ones in many historical pictures I've seen, has the thrust structure painted green under the engine fairings; I've often wondered why. A reader has recently suggested that it's zinc chromate, a corrosion resistant agent. A couple of Google searches seem to indicate that zinc chromate was used on the S-IC (at least in the forward skirt), so this does seem likely.

Note that the stringers which would be under the fairings are open-ended, whereas the stringers which are designed to be exposed are capped.