Produced by The Bristol Aeroplane Co. Ltd. Filton, Bristol. Cardboard 'computor' for the Hercules VI & XVI as used in the Wellington, Beaufighter and some Lancaster bombers. Used by the flight engineer to calculate how much fuel remains. He would select the RPM scale corresponding to the gear used and the boost scale corresponding to the mixture condition. The card disc was then rotated until the graduations appropriate to the operational RPM and boost coincide. Fuel consumption at the operational altitude could then be determined by a reading from the altitude scale that corresponds to the gear ratio. Simple! Some spot marking but in very good order. With original paper envelope. Stock code M16403.