I'm really really sorry, but I think you are incorrect about being able to put the boat together and getting decent section pc separation measurements. I do not know anything about these boats so I might be wrong, but hear me out . . .
I believe the boat shape is held by the gunwale, chine, and HULL_CENTRE stringers and that that hull centre stringer is attached together to form a long fairly stiff stringer and that the seat pieces essentially sit on top of it and the hull skin beside the chine stringers. This is the same format that all SOF's use. The seat pieces might also be a tight [but NOT essentially tight] fit between the section pieces . . .
Evidence of what I infer are:
- the centre stringer is not part of the seats - there are NO fittings for this essential piece, therefore I think it is separate
- there is no rigid attachment hardware from the seat base you have to the missing seat base.
- there is a pair of spacers [I think for the other seat pc also] that locates but doesn't attach to the hull stringer you have. ie because it doesn't attach, it isn't structural.
- the seat base stringers have curved ends to reduce/eliminate skin puncture - therefore they are discontinuous, therefore they are non-structural
- and I can see the shadow of the hull stringer here:
So could you re-look at how your boat goes together and see if what I'm saying makes some kind of sense. The reason I'm skeptical of what you say is that those seat piecs you show - do not show any stiffening attachments to forward and sternward pieces that would be essential for hull stiffness. I believe they JUST add some small helpful extra force outward, but are not essential.
Of course someone might chime in and help you out with specific measurements, but this looks like quite a simple pc that you could easily experiment with especially because they go UNDER the section pces and I think all you have to do is to EXPERIMENT with the placement of the outward pushing X_BRACES on top of the simply over-lengthened [ie gross measurements will work] seat stringers.
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so, as the mast was between the two seats, did it attach to the front or back of a section? ie was it closest to the back seat? If so it supports the idea that the seat you have is the front.