Yep, credit where credit's due, btt. I would not have thought about oil drain plugs unless you suggested it. As to the thread size - that's absolutely the right size for mine, I measured thread pitch and major diameter with calipers. After wearing out my eyes googling thread standards, I have no doubt that there are other possibilities out there.
The guy I bought the raft from in fact had a 3/8" pipe thread plug jammed in there - surprised the threads weren't destroyed, as you said.
Thanks again for your help, much appreciated.