All-Parts Fender Amp Skirted Knobs

Amp Knobs – Fender Style (2 pieces) Black, With Set Screw, fits USA Solid Shaft pots. Inside diameter approximately .25″ (6.37mm), outside diameter 1.183″ (30.05mm).

If you need an intro on the difference between split shaft pots/knobs and solid shaft pots/knobs, have a look at the photos.
The split shaft is on the left, it has a slot in the shaft, and the shaft is splined. These knobs just push on and tension keeps them in place.
The solid shaft is on the right, and it is smooth and round. No splines. The knob on this stays in place with a set screw.