New old stock Heavy Delta Rockwell Cast Iron Control knob.  Knob is approximately 3" diameter, is about 1-1/2" in height.  Bore is just under 1/2" and looks like a press fit, should be enough material to drill out to 1/2" and even carefully drill and tap for a set screw.  Nice heavy knob, great for machine restoration, or other projects.  Delta part number 426-06-027-5004.