[Alan Gales]

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.

Hasselblad and Rollie 6x6 projectors are not cheap like you said and the Mamiya Pro Cabin 6x7 projectors cost a small fortune!

I picked up a used Kindermann 6x6 medium format projector for about $100.00 off Ebay. They are quite common, inexpensive and use a tray system for 6x6 or 6x4.5 slides.