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What you observe (and that's a good catch) could be consistent with that - a bit of residual bulging, etc. where the major curve was met, the the fore-and-aft curvature had little place 'to go'. That could also be all the more reason why so many stiffeners may have been installed - to reduce buckling tendencies or more likely oil canning from such an installation.
Thank you. I can only wonder how does that bulge jive with the rivet patterns, both on the artifact and forensically seen in the pictures? One would think that underlying stringers would create more longitudinally linear patterns mitigating any such bulge. The stringers wouldn't have been that far apart.
First, as to 'that bulge', how real / prominent is it? Granted there well may be such a feature, but is is visually exaggerated by the light perhaps? See Ric's post above.
Second, if it is considerable - the stiffeners might brace that well, good point. Or they might not. Depends on how beefy they were and how they may have been conformed to the skin when shot in. They may have been shot in as an afterthought.
The surviving rivet suggests an aggregate stack-up of .060 according to the NTSB report on 2-2-V-1. That is, where that rivet was - and we don't know that all the stiffeners (or whatever was behind this piece) was that thick, others might have been lighter.
.060, if so, is fairly substantial, but again - how big is the 'bulge', truly (not so evident to me, seems to depend on photo angle / light), and how were the underlying pieces laid in? We don't really know. I tend to think the bulge may not have been so significant. See downstring -