Those were iron centerboards and they rust such that they swell against the sides of the trunk. They can be very difficult to get down when they do this. It certainly won't drop by itself. You will need to get some type of grip on it and pull it down - most likely with help of levers or hydraulics. If you can't get a grip, see if you can drill a hole in the edge and tap it to take a bolt. Bolt a loop of chain onto the board and use a lever to pull down on it. Use a hydraulic jack to push down on it if necessary.
When it is out, you will need to grind the rust sheets
off of it and decide if it is salvageable or necessary to make a new one.
BTW, that is a sweet little boat.