The piece with the slots goes INSIDE the neck of the shot snake. Then the other piece fits over that.
I installed one recently myself. I chose to use some epoxy to make the inner ring fit the leather neck. I used the slots to pinch in any loose leather. I used a hose clamp to pull the leather against the inner ring while the epoxy set.
After that I dry fitted the outer ring to be sure it would fit properly. I also wanted to index the head so it hung the way I thought looked best. Then I actually used epoxy to bond the outer ring onto the leather.
Normally I see these attached by drilling a hole from the outer through to the inner ring ring - then tapping the hole. Connect the two with a brass screw. I was a bit lazy and just let the epoxy hold things together. It’s working fine so far.
If it ever needs to be removed, a heat gun will loosen the epoxy and it can all be disassembled.
YMMV I am no gunsmith and not a skilled tinkerer. This was just my solution to the situation you are presently in yourself.