In the novel Destination Unknown, Hertz is a member of the staff working at Aristides's research establishment. The context of the conversation concerning him suggests that he is a plastic surgeon and that he comes from Vienna.
When Hilary Craven, posing as Thomas Betterton's wife Olive first met Bettertom at the secret research station set up by Mr Aristides, Hilary exclaimed that the man she met isn't Tom, isn't her husband. Tom had responded that it was the result of plastic surgery. "Hertz of Vienna is here. And he's a living marvel," Tom said.