Exposure meter, based on the photoelectric properties of a selenium cell, requiring no batteries.
The meter measures the amount of light, calibrated in EV (exposure value), then the outer gold dial
is rotated to align the EV number with the little red triangle. The exposure is read at the top,
in terms of the f number against shutter speed, for example f11 at 1/100th of a second.
Film speed is dialed in using the inner maroon ring, in this case set at an ASA value of 320.
[Picked up for a fiver (£5) in a charity (thrift) shop!]