King Ferdinand VIII.
Ferdinand VIII of Spain was a member of the Spanish Hohenzollerns who was King of Spain after 1851. Although Sobel does not mention his family relationship to Ferdinand VII, he was probably Ferdinand's grandson.
In 1853, Ferdinand agreed to serve on an arbitration panel to draft a peace treaty between the United States of Mexico and the Confederation of North America. Unlike his late elder brother King Miguel, King Ferdinand was eager to be a party to the peace negotiations, which he felt would add to his prestige. The panel began meeting in November 1853, and issued a report on 15 June 1855 that served as the basis for the Hague Treaty ending the Rocky Mountain War.
Sobel's source for King Ferdinand's role in the peace negotiations is Frank Taft's The End of the War: The Hague in 1853-1855 (Melbourne, 1967). In Chapter 13 of For Want of a Nail . . ., Sobel erroneously refers to Ferdinand VIII as Ferdinand VII.