I pulled this out of some piece of equipment I found in the skip. It has a rather unusual construction - at its heart, it has a "normal" 270° carbon potentiometer track. However, a cute ball-bearing reduction mechanism is used to give approximately a 10x reduction ratio. Three ball bearings are wedged between the outer casing and an extension of the shaft. The balls are held in a cage which is attached to the wiper. The action is essentially a planetary gearing system, but with the gears replaced by ball bearings.
Quick video of the shaft turning:
16/06/15: Here's the markings on the body: MEXICO 137-8818 AH9700. Unfortunately I can't track down anything based on that....