[broadgage]

Almost certainly for some type of vintage motor vehicle, and quite probably a stock item fitted to many different vehicles.

9 amps is rather small by todays standards but would have been ample back in the day.

The dynamos fitted to early vehicles were not regulated but did often have three different current settings, low, medium, and high.
The dynamo control was often interlocked with the main lighting switch.

Lights off=low charge rate selected.
Marker lights only in use=medium charge rate selected.
Headlights in use=high charge rate selected.

An additional control permitted the driver to select a higher charge rate if the battery was suspected to be low, due perhaps to frequent starts or low speeds.

With a pair of 6 volt 36 watt headlight bulbs, and perhaps 4 marker lights each of 6 watts, and a few small lamps for the dashboard, the total demand would be about 18 amps and the High dynamo setting would be about 20 to 22 amps, so as to give a slight charging current.

Note that the actual current was not sensed, only the position of the lighting switch.
Prolonged running with the lighting switch set to "headlights" but the lights unlit due to failed bulbs, could cook the battery.