It is part of a fish pond pump. Yesterday, the pump which feeds the filter and waterfall of our pond had stopped working. This is usually due to a small power 'blip' which knocks out the circuit breaker for the electrics - you can't be too careful with water and electricity! Normally it restarts when you reset the breaker. Yesterday...no.
Changed the fuse - no. So I stripped the whole thing down as far as normal and found the rotor was stuck solid. Further fiddling about got the rotor assembly apart and there was the culprit.
Trouble is, I don't know what it does! It fits in the hole near the centre of the face plate (the one at 7 o'clock) and generally flops about. All I can think of is that it's to stop the motor accidentally starting up in reverse when the power is applied.
Assuming this was its function I figured out that it wasn't strictly necessary as this was an emergency, so I reassembled it and it's working just fine. Perhaps I'd better try and get a spare though - just in case.
The worrying thing is that had we been on holiday for example, we could have lost all our Koi Carp - as well as the other fish.
I will try to get a decent picture of the grateful fish later...