I am hoping that someone can help identify this kill switch. It is off a Robin, but I believe that it was fitted by Kennard's hire when they had the engine in use (it is not a Robin part as far as I can tell). You can see a video of another Kennard'd engine here with a rather new looking one of these.
There are/were some numbers on the rear of the switch, but only the 96 can be made out now.
The special thing about this switch is it is a "press and latch" switch. This means that when you press and release it, it closes circuit until the engine stops, then resets to open circuit once the engine stops, so it can be restarted again without having to remember to turn on the ignition. Obviously it also removes the need to press and hold while the engine stops.
It still works well, but I would like to try and get some more.
Further information that I have come across: it is possible that this switch is a Robin part - used on their engines intended for use on fire fighting appliances.