Just got an RB67 Pro with Pro S 120 film back. In the photo below, there is (I think) a little piece missing that should hold the film spool in place (where the arrow points to). 

[![enter image description here](https://i.stack.imgur.com/VFjSL.jpg)](https://i.stack.imgur.com/VFjSL.jpg)

Right now, the film kinda wiggles around when loaded. There is something on the other side to hold the empty reel in solidly. 

Just wondering what this potential part is so I can find one.