Before there were StUPes, there were paddleboards.
Still a pretty big deal in the lifeguarding community. Long, thick, not terribly wide but ( unlike a gun) width carried well back with the pads a ways back there for knee paddling. Not a lot of rocker, dead straight flat in the back third to half. Fin way way back.
Sound familiar? Thought so. Have a look at some Marine Rescue is carrying ( my old friend Herb Rego’s company, nice guy) . The new ones are a bit different, some round bottom types, some scooped deck types, but the boards that look like yours are still well represented. In some cases they have interesting rules for particular competitions. Some very interesting stories there…
https://www.marine-rescue.com/rescue-equipment/competition-boards/competition-boards.html
https://www.marine-rescue.com/rescue-equipment/water-rescue-equipment/rescue-boards.html
Tandem boards tended to be very wide, 24" or better, there were special h giant blanks Clark made for 'em, round nosed, essentially like somebody put a Weber Performer on growth hormones. Some strong guy would catch the wave, turn a little and then do sorta gymnastic poses with a very teeny tiny light weight girl while the board was more or less staying pointed, possibly a cutback but usually just a Waikiki type ride. . The names I recall are Pete (Preston? Sammy, help me out here) Peterson, Dewey Weber and Mike Doyle, the women ( again, memory may be chancy here) - included Joyce Hoffman and Jericho Poppler.
But again, I really don’t think this is a tandem, shape is all wrong.
Big wave guns…didn’t look like that. I think they went shorter, pintail and pointed nose pretty much all the same time. Rocker all the way through, rails more positive, and again, no knee pads.
hope that’s of use
doc…