While this may look like a standard-issue 11mm wide single-shot slide knob, it's unique in that it's made out of a conductive plastic material. That makes it a great match for potentiometers that have capacitive touch detection on the slider such as the 5V Motorized Slide Potentiometer we stock in the Adafruit shop. 

Note that if you don't need conductive material, these nubbins are much less expensive and will do a great job.

Note: Potentiometer not included.