Plastic push-on control knobs with colored plastic inserts/indicator. These are the same knobs as found on the older Marshall pedals like The Guv'nor, Morley, and Award Session pedals.
These fit 1/4" (6mm) knurled shaft potentiometers
Pattern: Ribbed threaded knurled
Max External Diameter: 15mm
Top Diameter: 11mm
Shaft Hole Diameter: 6mm