I asked a couple of guys on the Boat Design Forum for their thoughts re your photos above; one came back with the following comments. He is a very experienced power boater in Australia :
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G'day, Off-hand I can't think what the likely build of it would be, for some reason I thought of Halvorsen in Sydney, which was a large builder of wooden craft back in the day, and including some sports fishing boats. The best clue I could offer, would be to enquire of this gentleman, Peter Bristow, who was a pioneer of the big-game fishing scene ( Black Marlin) at Cairns, North Queensland, way back to the late sixties, I suspect he would be as likely as anyone to be knowledgeable about that boat, or the most likely Australian makers of a boat like that. He is now located in Madeira, apparently, and is a pretty good age, but might be able to help with this.
Madeira Fishing with the "Katherine B" Sport Fishing boat. Madeira Fishing with the "Katherine B" Sport Fishing boat.
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Do you have any photos of the boat outside the shed?
Or if not, is she a classic sportsfisherman type (the transom door suggests this), or is she more of a motor yacht type?
The gentleman in Oz also came back with the following thoughts :
(we don't appear to be able to send you a private message yet - maybe because you have just joined?)
Our Aussie friend said :
There would have been a fleet of game fishing boats at Cairns in the years Bristow was there, I'm not sure if there were locally built boats among them. Port Stephens in NSW was another game fishing port. There would be quite a few yards that might have built such a boat in Australia, and if a Halvorsen, I would expect that to be the first thing mentioned, as it is something of a "name" brand. But I think Peter Bristow is worth a try, few would have the experience he has. I recall my father telling me of a conversation he had with him way back in the 60's, when he was at North Stradbroke Island, he sounded pretty approachable back in those days.
Edit - he later added this :
This chap would possibly know something, not sure if readily contactable, though.
He ran a marlin fishing charter business not far from the Gold Coast till fairly recently. https://au.linkedin.com/in/ken-brown-10b74689