Probably you want the back for 3 1/4 x 4 1/4" (8.5 x 10.8 cm) polaroid/fujiroid pack film. I have this back and alas it has no model number on it, but you can probably look it up at keh. The main distinguishing characteristic of this back, compared to the analogous rb one, is that it has a circle surrounding the square image frame, and this circle cuts (very annoyingly!) into the image a little bit on all four corners. The rz back can also be recognized by three metal contacts and an ISO knob, whereas my rb one does not. Oh and the rz one says rz on it
On the rb, I sometimes shoot 4x5 polaroid instant sheets. You can get a 545i back and do that on the rz as well, but... unless you have a stash of 4x5 polaroid or want to shoot some quickload squares, it's probably not worth it.