He pretty much summed it up. Both my Mamiya backs are Pro-S and 6x7. For me, the SD backs aren't worth the price, but I'm broke as a joke, so of course, right? Some people may find that they need them, but I'm more than happy with the Pro-S. I actually could use another 120 back, but I'm able to survive with what I have.
Originally Posted by MattKing
And nobody else has mentioned this yet (possibly because they don't want the secret to get out), but the RB67 is able to use Graflex roll film holders. Not the 4x5 ones though. I just got a Graflex RH-12 from someone here on the site for a steal. Works great too. They're usually much more expensive and hard to come by. I got lucky. But the Graflex RH-10 (which is 6x7) is surprisingly cheap. Not nearly as in demand as the RH-12. It's a possibility if you need another back.