Bought a compendium for my RB67, at least that was what I thought. It was an impulse buy and I should have looked at the pics better. When I got it today I realised that it was not for the RB but for the 645. The retaining ring is a ø67 mm, and the RB uses 77. The biggest difference is that this compendium is Rectangular, which means I will have to turn the compendium 90 degrees when I revolve the back, or pull the bellows back a bit. So what to do? Use it anyway with a 67-77 step down ring and risk vignetting or pass it on? I like it because it has two filter slots, and it has a real PRO look, which my plunger rubber hood hasn't. What would you do? I only paid $35 so it's no big deal really. But I have a policy about gear, what I don't use I don't keep.