mine was the same but did not have the cutouts on the ring. the nubs created their own seats when pushed on hard enough. make sure you are finished under there as you dont want to pull it off again unecessarily. disconnect the battery first and be pretty firmapplying as even pressure as you can get. I just put a timber cutoff over it and pushed hard. its never come off.

if its the push twist type i'll just shut up now

 

You might want to disconnect the battery when mucking with the horn. I used a tiny screw driver to hold the fiberglass disc up while pressing down on the horn cap. If you're shocking yourself, you don't having something wired correctly.