I figured maybe the grub screw ones would have a metal collet or something while the push on ones would have nylon rings or something softer to grip the splines... not sure though. I’ve sent an email to Fuzzdog.
In my experience, push on are splined to grip the splines and grub screw ones are made to accept the shafts that have one flat side. I'm sure they're not the only options though!
Yeah, if you don't have a flat edge on the shaft you should be able to nip the grub screw tight enough for general handling. If you want to abuse your knob you might find it slips round though. Many innuendo etc.
I figured maybe the grub screw ones would have a metal collet or something while the push on ones would have nylon rings or something softer to grip the splines... not sure though. I’ve sent an email to Fuzzdog.