More thinking required.
It was mounted under the dash top & has a small motor (about 1" dia) connected to a pump, when on it creates a vacuum in the pipes, there is a non-return valve to hold the vacuum, a solenoid operated valve to draw air from one of two circuits & a pressure switch which shuts the motor down when a vacuum is achieved. But it's so small I have no idea what it could have been for. Maybe something to do with the Supercharger? Some pre-oiling device maybe?
It weighed nearly 300g so it's gone anyway.