I'd never considered it might be a space for a battery. The PO did seem to have added quite a number of his own "touches," so that's a possibility. I don't know if you had time to watch the video I posted of it, but the cavity actually has a channel that runs all the way to the transom, where there is a drain hole (one of the ones that need to be plugged up with a rubber stopper, like the ones above it from the bilge.) So...I don't know what the heck, or when the heck, it was installed. I find it unlikely that the factory designed and installed this sketchy looking hole, but I find it just as unlikely that the PO designed the entire thing around a cavity that runs to the back of his boat and also installed his own drain hole perfectly centered under the bilge drain holes. From what few pictures of the Sailstar I can find online, it looks like most boats don't have this cavity, and instead have the raised piece that the mast sits on go all the way back to the centerboard, whereas mine seems like it was cut midway in order to leave space for this cavity.
In the video I also show what may be the fiberglass sheathing around what maybe should be a stringer but is just...empty. Shiver...
Now that you mention it, I far prefer to just glass over the whole thing, and maybe it's drain hole in the back too, since otherwise there's a chance the drain could fail and I'd never know because water wouldn't bubble up into the cabin like that time wife and I didn't know that hole needed a plug. So, to fiberglass it, I'd cut into the laminate on all sides at an obtuse angle, and then laminate some equaivalent-thickness plywood (on both sides?) and thixo/gelmagic it in, and also overlap fiberglass from my patch onto what I'm patchign onto? And then for the transom hole, same thing but with a little circular piece of wood, like I did for my viewing-port hole I made earlier?