A plug in circuit breaker for Wylex Standard fuseboxes, manufactured by Stotz Kontakt.
The larger button is to set the device, the smaller one to reset or disconnect it. These were common in the 1980s, later versions have a lever on the front.
This example is a 5 amp rating, with a white button.
The small red reset button is loose, only held in place by the mechanism behind. In the lid, three metal plates at the top form an arc supression chanber.
The long bar at the top of the mechanism directs any arc from the contact into the arc plates.
Connections are line in on the lower terminal blade, line out on the top blade.
Tripping occurs when the flat bar above the coil is pulled down, either by the magnetic coil, bottom strip bending downwards or the red reset button pushing against the angled section at the front.
The video shows the internals of the circuit breaker and several tests are done by passing a controlled current through the MCB so that ther thermal element causes it to trip.