Early in World War 2, a need was established for a quiet method of charging batteries for clandestine spy radios whilst operating in enemy territory and where no other supply was available. A steam operated set was put together by Stuart Turner of Henley on Thames, (Marine and Model Engineers), using a pre war designed model steam engine, direct coupled to a 6 volt 3 amp dynamo and fed with steam from a pressure cooker type boiler held in a brassier type fire basket. This was known as the Firefly and although successful, was bulky and not convenient to move in a hurry.

Stuart Turner then designed and built a much more sophisticated and powerful set, able to pack neatly into a crate measuring 28 x 15 x 15 inches and weighing approx 100 lbs (45 Kgm). The whole unit was designed for delivery by parachute and was designated Mk 814.

The single cylinder piston valve engine was combined with an inbuilt alternator with a metal rectifier giving 6 volts at 8 amps. It also had a unique variable stroke water feed pump and an integral injector pump for supplying the cylinder lubricating oil. The engine was built into a steel cradle with a canvas-carrying handle, together with the spanner for connecting the pipe-work.

The boiler set was in three sections: the grate, firebox and horizontal cast aluminium boiler. Fitted to the boiler were a heavy-duty water level sight glass, a pressure safety valve and a super heater coil to supply steam to the engine. The chimney was in three sections and incorporated a connection for the steam exhaust from the engine to keep the fire burning to maximum effect. The working pressure was set for 60 lbs/sq.inch. All connecting pipes were colour coded to ensure correct connections.