It's the Mid-Fi knob for 18T KNURLED SHAFT potentiometers (formerly known as the Armbruster knob). This sleek, shiny aluminum knob eliminates the need to mess with teeny tiny set screws. Just press them on your KNURLED SHAFT potentiometers. A sturdy plastic insert gives them grip.
Black aluminum knob
Plastic insert for 18T knurled shaft pot
15mm diameter at the base
P.S.: Like the size but want a smooth shaft? Check out The Mid-Fi knob.