That one is very very simple to determine the f-stops on:
The lens focal length is 300m, that is clearly marked on it.
The f-stop is the diameter of the effective aperture divided by the focal length - or to get the f-number, take 300mm/diameter. Since there is no glass in front of the aperture, the effective diameter of the entrance pupil is identical to the diameter of the aperture opening - as a circle, but the aperture is close enough to a circle to make the difference negligible.
Simply measure the size of the opening, and divide 300 by that number in mm. I have a cheap plastic measuring thingy which is perfect for the purpose - I think I bought it in Clas Olson.