Your "thru hull" in the keel stub sounds exactly like what we have in our 1979 I28.
As to the garboard plug, I wish I had gotten it installed last fall. This year we have the mast down so the boom cover isn't on the boat. My tarps, two layers, failed over the cockpit. Before I caught it the keel stub became full of water which is now frozen.
This boat is getting a plug! Next year, to be sure, I will also set some heavy bags of antifreeze jugs in the bilge, just in case of the open plug freezing over.
Three floors were already broken and coming out this spring so I'm not overly worried about the freeze up breaking them...