Just got a 203FE with this prism. There are these to button-like things, one on each side of the prism. I assume these are to be used when you want to dismount the back, however, only the one on the right side will let itself press down. The other one won't move anywhere at all. I'm just about clueless dismounting-wise right now. It's probably incredibly simple, but I can't figure it out... So my question is: How do I dismount the back when using the PM-90 prism? Thanks!