1/4 inch or 6 mm ? measure it accurately ! these knobs do not have a set-screw and rely on the grip.
there are four flavors of this damn thing
1/4 round and 1/4 d-shape
6mm round and 6mm d-shape..
it's gotta be right ...
Hi free_electron, I just received my supply. Here is what I am looking for a knob to fit a 6 mm shaft diameter / D-shape pot. The knob itself is 5/8" in height, 7/16" in diameter at its top and about 11/16" in diameter at its bottom with an inset D shaped hole about 6 mm in diameter. The part number is "0370-3238 KNOB 6mm." If you've got the 1/4" ones, they will work too. I tried both sizes from other supplies and the 1/4" ones are just a little more loose, but still function OK whereas the 6 mm ones are pretty snug on the shaft.
It's very hard to measure the difference between 6 mm (0.23622") and 6.35 mm (0.25000") with anything short of a vernier/digital caliper, especially since there are part tolerances to take into account and a third of a millimeter is quite small.
Here are some pics I took of an HP D shaped 1/4" knob that I borrowed from one of my other supplies, a 6653A:
Top/Front view:
Bottom view: