All I can tell you is that on the 17 and the 20, the compression sleeve drops down from the top, not in from the ends. The top hole is large enough for the sleeve to pass through. I think it would probably be stronger if it wasn't set up that way, but that's how it is. It would be a bit of a trick to get the sleeve in from the ends and lined up with the dolphin striker holes, especially on a curved crossbar.
You might not actually need to remove the dolphin striker rod (V bar). Since on the 18, the dolphin striker rod doesn't pass through a hole in the dolphin striker post (as it does on other models), instead it sits in a sleeve so you can remove the post without removing the rod. Just drill out the rivets on the mast step and pull the mast step and striker post up out of the crossbar. Then install the new dolphin striker post into the crossbar and down into the sleeve. I'm not sure if it'll work, but it might.
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