Bourns 500k Audio Taper Push Pull Pot - US Thread and 18 Spline. Includes Nut and Dress Washer. Used to control pickup phasing series/parallel switching coil-taps and more.
The threaded portion on the shaft is 10mm long... total length of the shaft to the top of the spline is 19mm. Diameter of the threaded portion of the shaft is 10mm. Diameter of the Split Shaft / Spline is 6mm.
Rule of thumb - 250K pots are generally used with single-coil pickups 500k pots are generally used with humbuckers.