Mamiya did offer a 6x4.5 back for the RB67 -- not sure but it might have been built in 220 as well as 120. As noted, there's little sense in a 6x6 film holder for a rotating back when the rotating back was the big deal, especially since the Graflex backs for 2x3 would fit. I've got a Graflex 22 for mine -- knob wind, and as with the RH12, no interlocks, but it works as long as you don't forget to pull the dark slide and seemingly a little easier to find than an RH12. I've also got an RH20 (6x7 on 220) and a Graflex 23 in addition to my Mamiya 6x4.5, 6x7, and 6x7 on 220 backs. All of these work on my Century Graphic as well, though the dark slide interlock on the Mamiya backs requires using the override tabs to remove the dark slide.