It probably should be open but since someone plugged it, I suspect that there may be an issue with water running forward from the aft bilge. We know that those hulls tend to collect water towards the mid bilge area so may be the previous owner wanted to stop that. Another possibility is that the previous owner would winterize the mid bilge area with antifreeze and plugged the drain tube to keep the antifreeze from draining out. That is an interesting idea assuming that water doesn't get in there when the boat is stored. If it did the mid m=bilge would eventually fill up.
In any case, pull the plug for now.