Selenium cell, simple D'Arsonval meter with calibrated resistance, and the printed scale. Likely adjustable (to calibrate) only before closing the housing. "Sears color film is ASA 25" -- that's Kodachrome from the 1950s. And it's for fixed-exposure cameras, or it would have speed/aperture scales. Likely sold with (or on the same catalog page, with "recommended for") one of the simple "family snapshot" cameras they pushed from around 1955 until 1970.
The most intriguing meter I have seen.
It is based both on the speed point and latitude of films. But... it does not designate the apt camera models, not even vaguely.
Basically a meter to say when better not to release a box camera, which themselves are based on film latitude.
It is a sophisticated GO / NO-GO Gauge for light based on ASA and typical camera configuration (i.e., shutter speed and aperture + as AgX says film latitude).