picked up a strange lens the other day

Marked Steinheil Munchen no. 40799 Patent

physically it is 3.5" tall and 3.75" total diameter..3" glass diameter

4 strong reflections

the weird part is the aperture is marked in the old U.S. system - down to f1/2 (as shown)

I kinda sort of calculated it out to be 16" or 17" and with an apparent aperture wide open of 63mm or so at the front glass

in no way does that = f2.8 or whatever f1/2 is..so

what is up??

it's kind of a nice size between the 14" and 20" lenses I already have though and will shoot it next shoot