The best Polaroid, ( now Fujiroid ), back for the
Mamiya RZ is the Pro II back.

Here's why;

The Pro II back has both black Horizontal & Vertical
6 x 7 Cropping Slides available, as well as a place to
store the normal shiny dark slide.

The Fujiroid back does not rotate & thus will always
shoot a 7 x 7 image. The cropping slides make it
easier to match the cropping of the Fujiroid to the
film image.

I always thought that these were brilliant, except
that while Mamiya, goes out of it's way to provide
a locking back cover to keep their film backs clean,
they never made one for the then Polaroid back.

I finally had to make one myself.