Shifter knob, Rock Nobs

[6pack]

I want to get one of these and am trying to determine if it will work or not. Here's a link to them: http://www.rocky-road.com/rocknob.html The ad says they are for screw-on knobs only. I've been told ours slide on and have a set screw that holds it in place. Is that true? And if it is, how do you take it off? The company says it should still work if it has a round post no larger than 14mm. Can anyone confirm that?
Thanks for your help!