I need help to work out the aperture settings for a vintage lens, believed to be approx 1915. The lens is marked:

H Duplouich, Paris, Rectilicne Extra Rapid

That is all: no max aperture and no focal length (my guess for the latter is 100mm).

The aperture scale runs from 2 to 22 with equidistant segments. 2 is the smallest aperture, 22 is WIDE OPEN, so it is not related to an f-stop scale. I think it is the aperture diameter in mm.

An idea how to work out f-stops?

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