If your 81 is like my 82 you should be able to lay on the port side berth and look down into the area of the hull aft of the centerboard. the water tank is in the center blocking that view but there is no liner in that area. Also by lifting the cover under the cushions you can see quite a bit.
The area under the sole is all lead so you can't see anything by removing the sole.
Do some measuring from the outside and see if you can match up the area that way.
Good luck. Let me know what you find.
"Beyond here there be Monsters".......Captain Barbosa