The original design was from Douglas aircraft in the early 1960s for a "Sombrero Hat" style fold out structure that could be used either as an orbiting station or a lunar habitat. The idea is the main structures would be launched as a nested segment affair, then folded out to become a complete structure which could then have equipment bolted into it. Douglas built several models for use in proposals to NASA and the Air Force and somehow Gene Roddenberry got ahold of one or two and those were used to make the segments of K-7. As I recall, Richard Datin commented that the original model pieces were made from a slightly rubbery plastic (which makes sense, given the materials that desktop models were made from at the time).