According to the US Holocaust Museum and the Amsterdam City Archive Cards, Walter and Betty Jacobsthal had two children, Joachim Adolf and Hilde, both of whom survived. Hans Joachim was born ten days prior to Hilde, so cannot be their son, perhaps a nephew? The story of their survival can be read at http://digitalassets.ushmm.org/photoarchives/detail.aspx?id=1160129&search=hans+jacobsthal&index=24
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