Knurled knob.
22mm (ish) diameter
for 3mm hex bolt, 5.4mm A/F
15mm height.
hence the file name:- 22-3-15

You will need a 3mm hex bolt, of appropriate length, or...
stick a nut in it.
The 3mm hole is tight, the bolt may require some effort to push it through.

Not quite the "diamond" pattern you get with lathe knurling, but close enough at present.

EDIT:- 16/12/19 - 4, 5 + 6mm hex bolt versions added.
2nd edit:- 14/11/23 - Found the 5mm version didn't work with some recently purchased nuts. Original nuts must have been an odd size. STL file and Sketchup model had now been uploaded. Tweak your own with the sketchup file perhaps?.
"knurled knob 22-5-15 8mm AF hex".

It's part of a larger project.
But it could be handy for others, therefore it's got it's own spot on the internet.

Will probably make a 6mm version later.