Are sit-in kayak seats universal?
Some have back rests, some back bands. They also vary in width as some SINKS are very wide, and others very narrow.
Some seats are adjustable and can be moved forward and aft. In other boats, they're in a fixed position.
They are universal in that you can sit on them inside the kayak.
Even different models from the same company or even different years of the same model are not at all similar or can be swapped. No more interchangeable than auto parts, I’m afraid.