The Uniform Scale (or US) is an old way to give the aperture settings of a lens. The following table gives the correspondence between regular f-stops and US stops:
The Uniform Scale was introduced in the 1890s. It was still used by some unscrupulous manufacturers in the 1950s, to make the lenses seem wider than they actually were: for example a lens marked as "US 3.5" has about f/8 maximal aperture. (See the Koniken for an example.)
- According to this post at the Large Format photography forum.