The postal Law : (english version) / Ministry of Communications Directorate General of Posts
Diccionario de la legislacion vigente de correos : [Espanha]
Postal Laws and Regulations of the United States of America : Published in accordance with the act of congresso approved March 3, 1879; Arthur H. Bissell
Memoria de la direccion general de correos : desde 1º de enero de 1879 á 31 de Diciembre de 1880 / Direccion General de Correos; Introdução Juan Penalva