Hi, On a normal vintage Flyer they are just simply screwed on, and usually with a 1/4″ or a 5/16″ coarse (Whitworth) thread. Maybe a previous owner has used Loctite, Superglue, or Araldite on the threads, or there may be a lot of rust on the thread. Wrap lots of turns of duct tape or insulation tape around the knob to stop it getting marked, and then unscrew with Mole Grips or something similar, like water-pump pliers. Don’t despair if it breaks in the process. Machinery suppliers keep them in stock for use on stuff like drill presses and lathes, and I have got a few tucked away too.