These fluted knobs are awesome. The 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