I've seen in pictures vertical poles on either side of the slide out table, directly under the mast. I can see screw holes where it appears mine had them at one point. My question is... Are these structurally necessary or not? It seems like they might help stiffen the deck up where it would probably need it.
Those support posts are necessary as they do support the deck . If you notice there is a steel plate under the deck running across under the mast foot area. Those support posts hold up the steel plate and provide stability for both the mast and anyone on the deck.
On my boat the steel plate is between the wood and the cabin roof. I very sure that this is original equipment. I don't have a headliner on my boat. I assume it was removed long ago if it was even installed. The plate is about 4" wide and approximately 3' long.
I know on mine it has the plate. Definitely agree with Dick, not only important but an absolute. If you were to step the mast, and raise sail, the slightest gust would probably dis-mast you. But if you look at the pictures on here, it is a fairly simple design, and should be fairly simple to reproduce.