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Reading through may (a jap imageboard) and I came across this
Apparently the hole is a thing that's required to fixate the bending joint (? I am a bit rusty in this application of the word so I might not be completely right, feel free to correct me) in place.
I did a little bit of research and apparently it's the same tech they applied with the SS OOO line. I personally have no problem with my SS OOOs but I have heard that that line has a tendency of joints being easily broken.