Varsity undercarriage.
As I recall ( long time ago),the gadget you refer to was the u/c hydraulic selector valve. Normally, the pilots used an electric selector button on the instrument panel, which caused an electric actuator to move the valve. If the electrics failed, removing a pip pin disconnected the actuator from the valve. The valve could then be moved to the required position by hand, and locked in that position. It was not normally used by the pilots.
I think it may have been the practice for the ground crew to lock the valve in the down position when the aircraft was not being used. I seem to remember that we had to check that the actuator was connected as part of the before start checks.