It's either reversed, or it could be a classic example of the AN aperture system.
There were several different aperture systems in use around 1900: The familiar f-stops (but as often as not with a different base number than 1, Germans liked the 4.5 - 6.3 - 9 - 13 - 18 - 25 - 36 - 50 series), Stoltze, "French", US, mm opening, 1/32" opening, and several others I can't remember at the moment.
But by far the most popular one (especially in the UK) seems to have been the dreaded AN. I have one very fine lens marked with 1-2-3-4-8-10, and through trial and error I have determined that these are entirely Arbitrary Numbers.