...and a heap of backs: 6x7, 6x6, 6x8, even a 6x7 Polaroid back...
The expensive, motorized, 6 X 8 back, from the RB line, will only provide a 6 X 8 image
when used with late model RB Pro'S' and RB Pro'SD' models.
The film plane opening on the RZ is smaller and limited to 7 X 7. The 6 X 8, RB back may mount,
with the necessary RB - RZ adapter, but physically, the RZ can only deliver 6 X 7 images.
Both the RB and RZ are still made...
The RB production line ended over a decade ago, and production of the RZ Pro IId,
the last of the new RZ bodies, possibly ended in late 2010...for 2011 delivery.
Camera retailers have a glut(?) of unsold NOS, RZ Pro-II's and RZ Pro-IId's on the shelves.
It is very probable that Mamiya is now only producing [new] 645-AFD cameras in MF.
Mamiya Press/Universals have not been made since about 1973(?).
As mentioned, repairs for these cameras become more necessary as every year passes,
but experienced, competent repairmen (and gals), are becoming even rarer then
quality, useable examples of these cameras. They're getting pretty, long-in-the-tooth.
The cameras, and the experienced repairmen.<grin>
If 6 X 9 is important...as mentioned, I'd look at offerings from Fuji.
680's and 690's were produced as late as 2008-2009(?), maybe later, and 612's and
617's were produced maybe till about 2006-2007(?). You will need to do more research on
Fuji's wide format, (panoramic) cameras. In Fuji, I only have experience with Fuji LF lenses.
Fuji optics (glass), is basically...second-to-none, or, 'really...really...good.'
Marc