Steve, here’s a pic of the Plymouth steering wheel cap from my 1967 Barracuda.
I’m not sure how the one you have attaches to the horn ring but this one just turns about an 1/8th of a turn to the left to remove it. There are three “spikes” that fit into the slots on the horn ring.
View attachment 1716245320
View attachment 1716245321
View attachment 1716245322
Edited for clarity