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.