Often the logo stamp "SDCH" is visible on this scrap metal, on interior surfaces of Japanese aircraft skins. What is the nature of this marking and its history?

Thanks to Ryan Toews for sending the answer to this question. SDCH is not a reference to an American company that sold scrap metal to Japan. According to Robert Mikesh, Japanese Aircraft Interiors, page 60, the Japanese company of Sumitomo Metals used this romanji script to identify the type of aluminum and its annealing state. SD stands for Super Duraluminum as opposed to ESD (Extra Super Duraluminum), both produced by Sumitomo. C means that it is clad with pure aluminum on both surfaces for corrosion resistance. H means the sheet is water-cooled after tempering at 500* C, cured at a normal temperature, then bent into shape. Other codes include SDCO to indicate annealing at 360* C for easier shaping, and then tempered. SDCR is material that has been rolled after being tempered.

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