What is significant about the gravestone that Tom and Isabel encounter on their way to Lucy's christening in Part Two: Chapter 15?

The memorial marker at the cemetery is seen by Tom and others on their way to Lucy's christening. Ralph's wife Hilda tells Tom and Isabel that the marker is for Franz Roennfeldt, who disappeared at sea with his infant daughter, Grace. This indicates Lucy's origins and also the fact that she has a distressed mother.