The expert has just relayed to me that the J12 cases have a ridge where one can fasten the movement with tabs, whereas cases that need heavy work because of tube replacement (like my Noob-based White Themes Franken above) might lose their ridge through drilling/milling necessary for repair, and therefore might require a movement holder (my White Themes Franken actually has a movement holder). He sent photos of both, like the ones we have posted, and insisted, like gashers says above, that the movement holder is not at all necessary mechanically when the ridges are there, and costs additional money and time.