This is the portable little brother to my Model 617; they do the same thing—this is just smaller (a lot smaller). This is a footcandle meter, which simply means the scale on the meter is calibrated to read directly in footcandles instead of a manufacturer's non-standard system. A footcandle is an actual, defined unit of measure and an industry standard.

This guy, like my 617 and my General Electric DW-40, appear to be used primarily for determining the appropriate light levels in a factory or an office. But if you have a conversion scale or a calculator dial, you can take the footcandle reading and use it for determining photographic exposure.

There are a host of those floating around. The one in the ad appears to be an early English version with a slightly spiffier case. Mine is a Type 6, which I suppose is late in the game. I have photos of the dial on the Type 3 and they appear virtually identical, save that the Type 3 has a ©1937 just above the pointer cutout, and mine does not. My guess is that changes between types are probably internal and minor.