The classic fluted MXR knob. It's got a brass insert and a set screw.
WxH= 33mm x 16.5mm
Brass Flat Head Set Screw
I've got 3 different sizes of classic fluted MXR-style knobs, from small (24mm) to large (33mm). Check out the measurements below for a size comparison.
Small: 24 x 15mm
Medium: 29 x 15.3mm
Large: 33 x 16.5mm
1/4" / 6.35mm smooth shaft
Beginner's Tip: The term "MXR" refers to the aesthetic design of the knob, popularized by the pedal manufacturer by MXR. Just because this is an MXR-style knob does not mean it automatically fits on all MXR pedals. The right fit depends on the shaft size of the knob's potentiometer. Read this post to figure out what kind of knob you need: https://lovemyswitches.com/news/what-knob-will-fit-on-my-gear/