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So I was a little bit unfaithful and asked if anybody found a cardboard under the foam in mytabletennis forum. One guy checked his aluminum Andro case and found the exact same construction with the same cardboard under the foam.
So the conclusions would be that my butterfly is genuine and it's kind of a common practice to make aluminum cases with cardboard. So they take a cardboard as a frame, add some thin supposedly aluminum (hard to tell without disassembling the case) skin on top to hide the cardboard, attach aluminum sides with cheap plastic corners and glue in the foam.