I just saw this (old) conversation.
Maybe I can help you.
I am the grandson of (Carl) Christian Fjerdingstad, the silversmith, who was born on Christiansø, a very small island north of Bornholm. (born 30 august 1891 / died 1968.)
His parents were Karl Peter Fjerdingstad and Helga Zoylner.
He had a younger brother, Robert, and a younger sister, Esther.
I can tell more things about the Fjerdingstad and about the Zoylner families, if you are interested.