I can't answer your question, but I can add to it. Some time ago I purchased a Series VII Protar (B&L) Cell----without shutter or the additional cell that makes a complete lens. It is marked 600mm. I thought for the longest time that I had wasted my money on this thing, but when I bought it there where other affordable (read cheap) protars on ebay and I thought that I would be able to find something for it to fit in fairly easily. Well, I never did and the cell sat on the shelf for a couple of years. Finally I built a lens board for it and made a box for the front where I can slide in front of the lens an aperture plate. The problem showed up when I put it on my 12x20 camera. It is not 600mm. It looks like about 690mm and it just barely fits on my homemade rails. It would be nice to know the exact focal lenght of this cell so that I could make better calculations for the F#. (But I guess it doesn't really matter as everything else about this camera is an approxamation.)
The really great thing about this contraption is that it has a tremendous amount of coverage. With maximum front and rear swings and front rise(with 12x20!!) I could not get anywhere near the edge of this lens cells' coverage. I have not let it focus on film yet, but will soon.