I haven't added additional comments into the 1975 specification page yet, but according to the "Corvette Black Book" by Mike Antonick, he states: "Paint quantities do not add to total production, possibly because additional units had non-standard paint, or primer only."
I don't know anything special about the codes, other than what the 'book' says. But, glad to see another '75 owner. Not enough of us around. If you get a chance, post some pictures of the engine compartment. I am collecting as many '75 engine pics as possible, in order to keep mine as original looking as possible.
I'm kind of unclear:confused on the question in your post but as far as rare interior / exterior codes I assume you mean rare combinations. If so, there are no known quantities for interior production in 75 except for the silver interior. All you can really do is look at what the Chevrolet recommended combos were and if you see one that someone ordered outside of the recommendations, it would be rare. Otherwise you're out of luck figuring out rarity.