Robert, the f number of a lens with only one cell can be found just by dividing the focal length by the diameter of the opening. (This doesn't work with combined lenses because it is not the actual aperture but the apparent aperture that counts..You have to measure what the diameter of the aperture appears to be from the front.)
I don't have a Verito to measure. Wollensak gives the lens diameter of the 14 1/2" size as 3 1/2". If the diaphragm opens that far, this would give an f number of 6.8 as the top aperture.
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