I spent a good part of my childhood racing and cruising on a thunderbird. Back in the 60s there was a strong class association with great regattas and social get togethers.
I was in Seattle recently and found a still active fleet and also visited Gig Harbour where I looked over the no 2 built. They were 26 feet so they could be built in the standard American garage. Ply because that was simple and affordable for the home builder. In fact the design was the winner of a competition put up by a plywood company.
I doubt many would have been glassed from launch. Maybe the seams in some of the early 70s built. Since the majority where home built the quality will depend on the builder and the ply. A fully glassed ply boat back then wouldn't have been competitive because of weight. I may be wrong.
There are so many cheapies on the market now of all designs that I'd prefer frozen snot over ply just to avoid heartache unless you have funds and it's an act of love to enjoy a restoration. After all you don't buy an old 66 beetle because they are better than 10 year old Corolla.
thunderbirdsailing.org/history/cheers