Hi,
I have 6X8 back on a RB67 ProSD. The 6X8 Back on RB67 ProSD gives a 56x76mm size negative in vertical direction and 56x73mm in horizontal direction. I get 9 frames per 120 roll. The back is only for 6X8 size; it cannot be used for 645 or 6x7. However 6x8 back can be adjusted to take 220 film. There is comparison picture with a 645 back in the link below:
http://www.flickr.com/photos/yazici/...in/photostream