These fluted knobs are awesome. The flat head set screw secures a brass cup to your 1/4" shaft that adds confidence boosting sturdiness. Check out the gallery pictures to see the brass cup for yourself. These knobs are for a smooth 1/4" shaft.
Some folks refer to 1250 knobs as "mini MXR" style knobs, and the similarities are there. Just note that these petite bad boys lack the skirt you'll find on a traditional MXR style knob.
Brass Set Screw
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/