[Godwin Hampton]

Stellan,as far as I know,

The Kerrison Predictor could work only with the Mountings Mk 1 and Mk111. The Mk1 was fitted with elevation and bearing receiver gears, which indicated to the layers where to point the gun by making a dial and a pointer coincide. This was the only way of "aiming" the gun, as no sights were fitted.
The Mk111 was the only mark fitted with the "Oil Units". The 2 layers ( for traverse and elevation) would unclutch their hand levers and dismount, leaving only the loader on the mounting. The Predictor would then control the gun via a cable plugged into a socket on the platform. This was connected to the oil units through a 15 way slip ring at the turntable. Data was fed into the predictor regarding the chosen target, giving bearing, speed, height, and angle of flight (horizontal or diving/climbing.)

The Predictor would work out a Future Position where hopefully the shell would meet the target, and worked the oil units so that the gun turned and elevated on its own to face the Future Position.

The gun was then fired by the loader using his foot pedal. Power to make the motors in the oil units turn was provided by a 2.75 Kw petrol engine driven generator, so this was also necessary in SP mountings. I suppose it would be carried in the Predictor vehicle.

All this became obsolete ( and simpler and cheaper) with the introduction of the Stiffkey Sight. On this the corrections were applied mechanically directly on to the layer's sights. The Sight Correction Number would control a handlebar with 2 motorcycle-type levers at its' ends. He would tilt the bar to match the angle of flight of the target, and would 'click' one of the levers a number of times,depending on his estimate of the target speed. After the engagement, the correction would be returned to Zero.
Thus many of the Mk111 mountings retrofitted with or produced with the Mk5 Correctional Sight are without the oil units.