Dealer said the same casting is listed for both Infusion and 20 (which sounded odd but I went for it anyway). It looks right (and same piece comes up with a picture on Murrays), but I'll do more inspection before drilling any rivets. I was wondering if there was a plug. I may still seal the holes.

Chris - if the mast is unsealed at the base it would certainly fill if the boat went turtle. I can also think it would help the mast fill if there were a leak near the top.