Well, I was just telling you how to tell the rb and rz backs apart, visually. You might find model numbers at mamiya usa, but I am saying that I don't see the model #s on the backs themselves, so whoever is selling them probably won't know what those numbers are. Check at KEH, they do a good job of sorting them properly. Anyway the back you want is for 3 1/4" pack film. And that film is currently available and very nice. Try fuji fp100b and fp100c in particular. You can do lovely emulsion lifts with the fp100c.