IIRC the stems have barbs on them. Push knob on and the barbs grab. Once you rip the stem out of a knob, the knob will probably need epoxy to re-use it.
Clamp the stem in a vise with soft jaws or backed up with wood so you don't distort the shaft under pressure. The last one I did, the knob pulled off the stem by hand with moderate force.
Good luck.