What family heirloom is Whitney given after the birth of her son in the novel, Whitney, My Love?

After the birth of her son, Noel, the future heir to Clayton's title, Whitney is presented with a wooden chest. This chest contains letters written by previous Duchesses of Claymore that contain the stories of their romances and the births of their sons. Whitney reads each one before adding her own story and a picture of herself that Clayton chose, as per tradition.