And I have one for my Mamiya 645 Pro - or at least the older style one that was originally designed for the older style bodies.
But both versions necessitate using a tripod and a prism finder - handheld use just doesn't work with a waist level finder, unless you limit yourself to landscape orientation.
One really can't describe how satisfying it can be to use the 645 backs on the RB67 - you have to try it to appreciate it.
And of course none of the 6x4.5 cameras offer a 6x7 option by just buying another back, do they?