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You are correct. There is no "Mitsiko" or "Miziko" in Japanese nor is it a possible combination of sounds.

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I think it is very likely that your Opa misheard either Mitsuko or Michiko. (In certain transcription systems which were formerly common but not much used any more, Michiko is written Mitiko, so it's also possibly that he saw the name written this way and misinterpreted it). Mitsuko and Michiko were both very common names during WWII and for some decades thereafter.