On my 275, the cabling to the mast goes under the locker above the Stbd berth, and into the heads. From there it follows the top of the aft bulkhead of the heads to go through the inboard bulkhead of the heads into the space above the keel box under the deck. When I replaced the carpet lining on the inboard bulkhead of the fo'c's'le, there was an access just under the deck through the inboard fo'c's'le bulkhead to the area where the cables penetrate the deck just aft of the mast.
Hope that description is clear - let me know if you have any queries.