It is designed to produce (AC) electrical voltage from rotational movement; it is obviously not a dynamo (alternator) but I hadn’t thought of an engine tachometer.

It is fairly big; it would just fit within a five inch (125mm) box. Also, in the photograph, it is sitting on two mounting ‘feet’; that made me think it was intended to be mounted on the airframe rather than the engine (but that is just guesswork on my part). It is heavy too; over a kilogram.

For some reason I thought it may be to do with radio equipment but I couldn’t say why?

Could it be from a very small APU (with a pull-cord start)?