TOPIC: Deck vents, or drain holes?

Proud new owner of a 1965 MFG Westfield Custom, with Merc 900. The boat appears to have been stored indoors most of its life--the gelcoat still has some gloss, the fake wood laminate is still adhering to the map pockets, and the deck is super firm, with no soft spots.
I have a few questions perhaps the MFG experts can help me with. First is about the 1/2-inch rubber plugs in the decking, as seen in the attached photo: are they original, and is their purpose to exclude water from above, or to vent vapor from below, or what? In the two or three cases where the rubber plug is missing, should I plug the hole or replace rubber plug (and where to source)? I obviously want to protect the condition of the structures below the deck. Many thanks!

Here is a closeup

As a 1965 Westfield Custom, your boat originally had the Sun 'N Fun seats. Check out the 1965 MFG sales brochure in the Fiberglassics Library for more details.

The hole you see with rubber around it is actually a well nut that was fitted at the factory to hold the seats to the floor deck with an aluminum bracket. The rubber sealed out water from getting into the space below the floor, which has fiberglass stringers along with foam for flotation. The drain plug in the transom to the right of the middle plug is for draining that area, in case it ever did get water inside.

Your boat has had the original seats replaced with newer back to back lounge-type seats. Make sure the previous owner used well nuts to secure the new seats to the floor to keep water out.

If it were me, I'd use some 3M 5200 (probably black to be consistent in color) to seal the old holes up, or perhaps remove the well nuts and patch the holes. Could be a bit tricky to match the rough beige colored floor - my 1966 has the same floor.

My 1966 MFG Niagara (base model) also used well nuts for attaching the standard back to back seats to the floor. They are still in good shape after all these years.

Thanks for your response, that makes sense. I will check the mounting points for the new seats to see if they are proper. (Incidentally, these new seats function like the Sun 'N Fun seats, folding out to the lounger configuration.) And I'm going to seal those open holes! Appreciate the assistance,