Normally, the top sections overlap the back sections, else the joints show when the timber contracts. Some narrower and less deep profiles can be moulded into a T&G board.
That could be slats fitted to a ply backing board going by the slat thickness and spacing, but the way some of it is locally stained the same (due to water soaking up in the end grain) suggests individual boards.
Keep in mind that if you go vertical, when the bottom part inevitably starts to go you have to replace the lot as opposed to just the bottom 1 or 2 pieces.
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