JPlomley, Thanks for posting that. I'm surprised to hear that a slim mount would vignette, does it do it at all apertures? Before you sell the 55 you might want to try a B&W Extra wide filter, which is made more like a center filter. The extra wides are basically a step up ring with a larger filter all in one unit. The center filter on your 55 is most likely a 67mm to 86mm, right? An extra wide filter for that would be 86mm to 105mm front threads. You could add a cheap 86 to 105 step up and see if that vignettes, before buying anything expensive. Maybe wrap some electrical tape (cut to the correct height) around the inside of the 105mm part, or something else, to simulate the added height and thickness of a filter mount and do some tests. You might find that a regular filter will work on the step-up ring. But, if a filter 3mm thick is vignetting, a step-up ring might add too much height and thickness where it attaches to the C.F. threads too. Hope you find a solution to that vignetting.
I just ordered a B&W extra wide filter (6-8 weeks special order) for my 90mm with C.F. I hope it works for 5x7, if it doesn't I'll at least have plenty of room to move when I use it for 4x5. I'll definitely post when I have a chance to test it.
Here's a link to an extra wide polarizer so you can see one.
I'm curious about you using a polarizer on such a wide lens, what kind of subject matter are you using it for? I've been thinking about getting one for my 90 & 65mm, but have read that they are of limited use with wides.
How do you like the IIIB C.F. on the 55? The IIIB corrects 1.5 stops, but according to the Linos (Rodenstock) website the recommended filter for the 55 corrects 2.5 stops. I just bought a IIIB for my Nikkor 65. From what I understand the IIIB works on the 47xl too, as well as the 80xl, 90/f8, and 110xl. It seems like it's the C.F. to own, since it is compatible with so many lenses.