If you mean without the flutes and concentric rings ... probably pretty simple for someone that knows how to use a CNC lathe. With the bling ... jeez, probably five axis CNC work.
Actually the file I created for that knob (to 3D print it) would translate fine to CAM software for machining. Easy enough to remove the fancy features.
But, how to machine the notched hole for the stem on a radio I have no idea. Unless you just went with a round hole and added a tiny set screw for the stem flat. 68 of course were injection molded plastic ... and the older metal knobs were probably cast.
However if you want crude plastic knobs in various colors of the right size and shape I can provide those
Or "custom" shapes, like hearts, moons and clovers
Here is a first attempt at printing one, I can do about 50% better, I was setup for printing "functional" parts at the time, not detailed parts. I'll spare you the details. The shiny black plastic did not photo well, I'll print out another before long in a silver plastic. Certainly not a piece a person would be proud to have in their vehicle ... but as opposed to none because a part couldn't be found ... it would work.
