The CB lens is a newer model than the CF. It is like the CFI (or is it CFE?) lens but it has one less element and no "F" setting for use with focal plane shutter cameras, so it was a much more affordable Hasselblad lens. Of course... an affordable Hasselblad lens isn't exactly that affordable. I actually bought the camera because it was such a great price. I used to have a 501c with a CF lens and I was quite happy with it, but I had to sell it to move up to large format.
I did think more about the shutter closing and after looking at a view camera lens, I thought that just must be how leaf shutters work (and you point this out when using the mirror lock up, essentially the camera is working like a view camera lens). However, the initial shutter closing is not "very rapid" as you desrcibe it (though I would still call it rapid), but when I look through the back of the camera, with the back off so I can see through the lens, I do not see the shutter close, open, and then close again, so even though I notice the shutter initially closing when I look at the front of the lens, that moment or so before it closes is not allowing light to hit the film.
Thanks for your response, and if anyone out there uses a CB lens, especially an 80mm one, I'd really appreciate hearing from you if your lens is the same.