SOFI coatings for the hydrolox stages + the S-1E, as well as an Atlas-V coppery texture for the S-1E. Variant textures are usually just "one of the team was bored that night and felt like it", so they're added whenever one of us gets around to them.
I feel like we're in 2016 again It's always been marginal - are you using the orbital config for the SM? You can also remove most of the ablator which takes off a lot of mass. Might be worth making that a B9 actually, so it's more apparent / we can balance more directly.
To elaborate - we're looking into how to combine the variant textures onto a smaller number of sheets. We used a similar trick for the Mercury variants - the white and orange are on a single sheet. Believe it or not, we DO try to be somewhat RAM conscious, especially with these variant textures.
I think C-3 is one of those ones that was really just a notional set of numbers. "Ok we have two F-1s, so the first stage should have about this much fuel mass to be optimal...", etc up the stack. You can get a lot of different configurations like that.
As for C-3 in BDB... well, it would be cool "some day", but in the meantime it's just another set of parts for another diameter that we'd have to flesh out, add adapters, decouplers, fairings, etc. So not something I'm in a rush to add.
It's the O2/H2 Burner (which was used to heat the helium system for pressurization); it would only be used on flights where the J-2 restarts, which, to be fair, I don't think the S-IVC in Eyes did. But I was just ripping the greebles off the base single node model and jamming them on.
Maybe we'll see these parts get used more? Always thought they were cool... but then again, I'm sitting at a desk with two Athena posters behind me