Weston Cine Exposure Meter Model 819: 1937, USA Manufacture.
This was the first meter marked up in units of 'Relative Brightness' - a simple linear numerical scale. The cine film emulsion speed is set on the dial and the camera type is dialled in. The pointer is set to the cameras framing rate. The aperture (f number) setting is then read off for the brightness reading from the meter.
Physically, this meter is an adaptation of the classic type 650 and retains its Art Deco bakelite casing. The camera type needs to be set since, although the framing rate may already be established, the exposure arc from the rotating shutter will be different for various makes of cine camera and may be adjustable on certain models. This setting combines with the framing rate to give the effective shutter speed per frame.