Carlo is a reference to Carlos María Isidro de Borbón (1788-1855), the brother of Ferdinand, from whom the Carlists took their name and whom they felt should inherit the throne.  Serrano is Francisco Serrano y Domínguez , duke de la Torre (1810-1885), who, after first supporting Isabella II, helped dethrone her in the Revolution of 1867. He served in positions for power following the revolution, but in 1875 Serrano was in exile, after the accession of Alfonso XII. He returned to Spain in 1881 (Holt). (Image: Marshal Serrano, Regent of Spain).