Ric- there was a projector made years ago for projecting 4x5 slides. At least I recall seeing one in a catalog somewhere. But it was rather ridiculous. And MF projectors aren't quite as rare as you think, but they are silly expensive. And the only ones that could handle 6x7 transparencies had a manual, one-at-a-time loading system. Only the 6x6/6x4.5 projectors had some kind of tray or carousel.