I think it was a submarine steering auxiliary motor, Vickers 4 cylinder.
From
dambusters.org.uk:
Once loaded and fitted in place the mine was initially intended to be spun by a Ford V8 engine but this was not implemented, Instead the hydraulic supply normally used for the mid-upper turret [removed of course on the Type 464 provisioning aircraft] was used to power a Vickers "Janny" hydraulic motor [originally believed to have been designed, for the steering gear of submarines] the speed of rotation being controlled by the W/Op who monitored a rev counter and controlled the hydraulic pressure via a control valve to a gearbox and pulley which drove a rubber belt and spun the bomb via a pulley. When the bomb was to be released two spring loaded arms which held it in place were allowed to flip outwards under spring pressure allowing the mine to drop