I came across this item in a box of random parts that someone gave to me.
It’s a potentiometer that was once part of an Erectronics breadboard system.
The Erectronics system was an interesting solderless breadboard. The plastic bases have pegs to hold the base into a sheet of pegboard. The components are then connected using wires that have spring clips that clip onto the metal pins.
Allan Jayne has a nice web page on the Erectronics system at http://www.cockam.com/erectron.htm. There has also been some discussion of it i this thread on the Antique Radio Forum: "Advice on origin of my GEL Vacuum tube breadboard system”.