Jacven

Hi Guys. I have a old cresta craft. While freshing up the inside over the lock down I found a purposeful hole in a box in section under the floor in the center of the boat at the back. The hole seems to be similar to the bung hole in the transom but inside the boat about 300mm in from the transom. The hole has a casing around it and it threaded. So my question is. What is it for and should it be plugged?