This is a popular knob with an arrow-shaped indicator. This style is best known for Dumble amps, but can be used for guitar pedals, amps, etc.

About 24mm (0.94 inch) diameter at the base and 14.5mm (0.57 inch) tall. Uses a single flat head set screw to fasten onto a 6.35mm (1/4 inch) potentiometer shaft. 

You get 4 knobs.