Here's what I mean, worded differently. Judging from the behind the scenes clip and the magazine photo (which is the same as that nice black and white photo earlier in the thread), all three joints/hinges look to have the same construction: one side is concave or scooped out, to fit over a convex ridge that is built into the mating face on the other half.
Both wing tips have a groove along their joints; the two body pieces both have ridges along the wing joint; at the central joint one body piece has the ridge shape and the other the groove. This central ridge allows the batarang joints to fold in either direction and self-locate and self-align when the internal bungee contracts.
My disconnect is that, based on the idea that all three joints are built similarly, when the batarang is expanded/unfolded the parting lines on the pointy ends of all three joints should look the same (with one joint mirrored with respect to the other two). See how the parting lines have similar geometry the reference/magazine photo (left and central portions flipped in editing for easier comparison):
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The shape of the joint necessitates that the convex/ridge side of each joint has a little more material, including the very tip of the point. The concave/groove side will actually come to two small soft points that straddle the ridge side of the joint when unfolded, but do not extend all the way to the tip of the ridge side.
Here is the movie close-up compared to the reference photo (I mirrored the left wing and central "tail" hinge in the magazine photo so that the orientation is consistent with the screencap central tail hinge).
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Note how the bright glare on the right face of the tail is interrupted, as is the glare on the front face (this is the body half with the groove part of the joint). Compared to the reference photo of the tail tip, this indicates that material is missing here (one of the two smaller points that are found at the end of the groove, as mentioned above). The separate triangular shaped glare is the tip of the tail that is part of the joint ridge from the other body half.
It looks to me that the concave/groove side has part of its lower end broken off, at the red line below. The yellow lines show the shape of the missing portion.
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Here are some additional screencaps from this sequence:
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The separate tail tip (part of the ridged side of the joint) and full thickness of the batarang is more clearly visible here as it picks up more light. It's part of the split end of the grooved side that is missing:
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I may have missed some useful frames as I can't do frame-by-frame slow motion. Is anyone able to screen cap all of the frames where the batarang unfolds?