Wall mounted dimmer switches from the 1970s.
The rear terminals include both a fixed and dimmed output, allowing some lamps to be dimmed with others at the full brightness.
This example has a crack in the front plate and significant quantities of white paint on the edges.
The dimmer is a typical triac/diac arrangement which controls the brightness by switching at various points during the 50Hz AC waveform.
Switches are rated 5A, dimmer maximum 500 watts.
In this video, the single switch is dismantled to show the internal components and construction in detail.