There isn’t an extra hole there, so this was added. Also, the original 356 does not have the “teak” cockpit. That was also added. If you buy the boat, I’d fill that hole so,you won’t possibly get a leak into the bedroom below. Not certain, but if it leaks there, water could eventually get to the interior. Was probably put there for a wheel pilot type autopilot is my guess. Also, I would not take a chance in the covered up goop. If me, I would discount the price and replace the base with a new one, but I would not want to fix if I was buying myself a “new to me” boat.
The 356 is a great boat. 1165 days on ours since new in 2003. We like the galley design better than the later 36 models. For cruising, the deep freezer and refrigerator in the counter is superior and holds more than the newer front opening refrigerators with tiny freezers. Also we like the location of the garbage can in our model - easier to access. This is one of Hunter’s best boat designs.