Beardy,
It was called a stick positioning device and consisted of a small drum of cable, the end of which could be clipped to the forward side of the stick top. As Lomcevak said, its purpose was to prevent inadvertant aft movement of the stick during catapult launch. The drum had a clutch which could be overridden by pulling harder if necessary to raise the nose in an emergency. During my apprenticeship at Hawker Siddeley Aviation Brough in 1970, I worked in Structural Test and carried out the endurance test on the device which consisted of operating it several thousand times by hand and checking the breakout load of the clutch at regular intervals. Not the most exciting way of pushing forward the frontiers of aviation.
Walbut