Looking at the brochure picture again. It looks like the transom was wood to the outside by design.
If so; No previous owner cut off the fiberglass transom...there never was any. You have a complete, original, molding there!
No, not correct, the Donzi Magnum had a fiberglass cover on it, there were quite a few different tunnels back in those days and different boat companies did different things with them, but I remember them very well and the majority of them had the fiberglass cover over the transom.
Donzi was not the only company making the Magnum Tunnel Hull, if you look through this google image link, you will see many different versions of the Magnum, I remember only one that didn't have the fiberglass cover and the transom was not plywood, it was mahogany and only lasted about 1 year. It had many coats of varnish on it and had to be re-finished about once every 6 months if it was actually being used.
https://www.google.com/search?q=Donz...miVXUaBzOKM%3A
To be honest with you, I am actually starting to wonder if this is a Donzi Hull, it is starting to look a lot more like the Glastron Molinari 2 + 2 Sport Runabout to me, but I could be wrong, the one in the posted advertisement really looks a lot like the '68 Brunswick Mercury Magnum Missile Tunnel with that outside mounted throttle control.
But like I said, there was a period of about 20 years, those style of tunnel hulls were real popular, the first one I ever drove when I was around 16 or 17 was a Cougar Tunnel with a really hopped up Ford 427 SOHC motor in it, which was the same motor that many of the NASCAR teams were running at that time.
In fact if I remember correctly and if I am wrong I am sorry, it was 40 years ago, Popular Science or Popular Mechanics sold the plans for the Magnum Tunnel Hull in about 1972 or 73 so you could build a version of this boat at home, kit boats were really popular back in those days and the Magnum Tunnel was one of the leaders of the pack.
This thread is starting to make me feel really old!