There is a knot meter in the hull at a separate location. It is not in the bilge but on the hull aft of the keel. That sure sounds like a probable answer. I have not found another knot meter but there are two more spare plugs, both with anti fouling paint
on their exterior surfaces. One has its O rings and the other one does not. That would be 3 dummies? I suppose it might have been a knot meter and was kept when for some reason another one was installed. I could use it as a garboard drain I suppose. I am not excited about having "extra" holes in the hull.