This is what site flush with the bottom of your tank? I don't think that is the right piece at all for that spot. The plumbing for my Cade 1200 looked like in this video below. The piece that goes flush against the tank (on the underside) is hexagonal, and as I recall flat with no O-ring.
Yeah mine didn't come with the hex fitting there, and the drain valve is different also so it's likely they updated/modified the previous set up a bit. Even still, would expect at least the larger flat washer/combined with O ring there. The Emergency overflow has it.