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I did not see in the picture a brass through drain in the hull below the water line on the starboard side of the hull. Do you have through drains on the boat? There should be several of them and when on a trailer would allow drainage. You mentioned "....the remains of a rubber drain bung (plug/tube?)......". Is this an intact drain tube? The brass through drain tubes are 3" to 5" long. I did see a Whaler (same style hull) where one of the previous owners filled in and sealed all through drain tubes. I recently was chatting with Chuck at BW regarding bilge pumps hull lay outs (I have a 1979 22' Revenge) and there was a factory option for bilge pumps in the rigging tunnel by the stern (factory plug location there) as well as in the deep chamber that is in front of the console. That chamber should have a drain tube on the bottom.