It really just caught my interest because I've never seen it. My guess is it wasn't sufficiently less expensive so people just got the model A instead. The eclipse design was after olympos. It's the most recent Japanese full airbrush patent. The changes made after olympos to the micron head/needle seem to indicate that katsuaki has a pretty good idea of what he's doing. Whether that comes down to actual calculation or trial and error plus experience is a different matter. Then of course some changes were simply made possible/practical by the advent of CNC and better tooling materials. Some for the better, some simply for manufacturing ease. As you've said though, really only minor variations have ever taken place beyond the wold a1.