https://www.mcmaster.com/products/knobs/ball-knobs~/thread-size~1-2-13/head-diameter~2/I found two at McMaster Carr. I like them a lot better than Granger for hardware stuff. They have everything and ship fast. One knob is smooth round and the other has a hash pattern around the middle. Mine had grip serrations around the middle of the ball. The McMaster Carr one has the same serrations. Although is it no completely round. I am getting both to see which one I like better and may still have my son create one exactly like the one we had on ours.
Does any else have a PTO knob that has the serrations? Both my transfer case and transmission knobs have the same serrations.