Thanks, Jimmy for the reminder. I have some of those lenses of 1900 vintage that are engraved with the old US stop system. I knew that they are not f/ stops and found a conversion chart some time back. By that time I had measured to determine what f/ they were. In case there are others who are puzzled by the numbers or are trying to figure out why their exposures are so far off here they are:

US 4= f/8, US 8=f/11, US 16=f/16, US32=f/22, US64=f/32, US128=f/45, US256=f/64