Ancestry.co.uk has a record of the naturalization of a Rudolf Siegfried Nothmann, born in Hamburg on 04/02/1907. It also has a record of the death of Rudolf Siegfried Nothmann in Los Angeles on 25/08/1997; the latter gives his mother's maiden name as "Heymmann" (sic) and gives his date of birth as 01/02/1907. It seems very likely that, allowing for typographical errors, these record relate to the same person and that he is a son of Nathan Nothmann and his wife Gella Henriette geb. Heymann who were married in Hamburg in 1905.