f 256?

I came across an old brass lens last week that has a range of f stops from f 4 to f 256. I though I remembered reading somewhere that the older lenses used a different formula to compute stops.
It's a real clean and I'd like to try it out on the 8x10 but not before getting a proper exposure setting. How would I convert these old stops to something I could use? It's about a 300 mm and f 256 measures out to about 3/16"
And while I'm at it...inked into the barrel is "Rapid Symmetrical 8x10" and "Hyatt Supply Co., St. Louis."

Ahy assistance would be appreciated.

This was passed on to me from another APUG member. I hope it helps.
http://www.largeformatphotography.info/shutters.html#apertures

Arthur

As the chart in that last link indicates, it's probably marked in the US system. The easy way to figure out those markings is to remember that US 16 is the same as f16. Count up or down from there and you can figure out what f stop you have without much trouble.
f4, f5.6, f8, f11, f16, f22, f32 equals US 1, 2, 4, 8, 16, 32, 64, etc. US 256 is f64

Rapid Rectilinears generally start at about f8 (US 4). The Hyatt Supply Co. was probably a local supplier of photo equipment that put its imprint on lenses made by one of the Rochester companies, such as B&L, though I could be wrong on that. I have one of these from an O.H. Beck company in Minneapolis

Thanks for those quick replies. I knew I could count on this forum for the answer.

One more question on this topic...when was the US system discontinued?