Compression posts are right under the mast between the keel and the underside of the deck, at least they are suppose to be.
At that point the deck should be solid fiberglass
or cored with solid wood
One of the problems with the old wooden boats is that area under the mast would rot
out and compromise the rigging. Todays boats are built so that area should be one of the strongest parts
of the boat.
One of the problems with keel stepped masts is that the deck around the mast will start to bow upward when heeling over.The shrouds start squeezing the hull
together bulging the deck. So, to compensate, a draw bar (brain fart-can't remember the name) is attached between the keel and a deck plate, keeping the deck from rising up when the shrouds are pulling on the chain plates.
Some boats have a bulkhead fairly close to the mast, which helps to support the deck and to keep the hull
from squeezing together. My bulkhead is 18" + or - but I still have the draw bar as extra strength. But when she's heeled over 35 degrees I can see the deck around the mast moving. I can see the boot working up/down. If I take this boat offshore
I'm cutt'n the mast and steping it over a mahogany post. One hand carved like from the Philippines
BTW the base of a deck stepped mast should be trapped by a casting made to fit the mast and bolted to the deck.