Over many years, professional photographers the world over have user Weston exposure meters. Why? Because they fulfill the professional's needs. Extremely accurate in all lighting situations, rugged, generally no reliance on batteries.
The epitome of the selenium cell meter is the Weston Master, which has a long and complicated history, not least because there were both US-built and UK-built versions: the UK company started as a subsidiary of the US-based company (which was founded by an Englishman) and became 51% UK owned (as Sangamo Weston) in 1936.
The Weston Master Universal was introduced in 1939 and remained in production in the UK until about 1950, though the Weston Master II was introduced in the United States in 1945/6. The Weston Master III appeared in 1956; the Weston Master IV in 1965; the Weston Master V in 1967; and the Euro-Master in 1970. Even after Weston lost interest, the Euro-Master was manufactured by Kilbride Instruments in Scotland from 1980 to 84 and then as the Euro-Master II by Megatron in London from 1984 to 2010.
It works as follows: moving the small tab that sticks out from the silver dial adjusts the film speed setting which can be read though the small opening in the red dial. Then take a reading by aiming the meter cell on the bottom of the meter at the subject. If the light levels are high, leave the perforated cover over the selenium cell in place; if the the light is dim, then open the perforated cover. The meter needle will point to a number on the meter scale. The range of numbers for bright light is 25 to 1600 and for low light it goes from .2 to 50. Use the turned up tabs on the black wheel to aim the large silver arrow at the corresonding number on the outermost part of the dial. You can then read all the correct exposure combinations off the silver (shutter speeds) and black (f stops) dials. In other words, a shutter speed that lines up with an adjacent f stop should provide the right exposure.
Manual is here