^ 2.02.1Carlton, R. Scott. The International Encyclopedic Dictionary of Philately. Iola WI: Krause Publications, 1997, p.117. ISBN0-87341-448-9.
^Van Gelder, Peter J.; The Collectors' Guide to Postal Stationery, A Squirrel Publication, Shrewsbury, UK (1997) ISBN0-947604-07-3
^Ewen, Herbert L'Estrange. Priced Catalogue of the Unadhesive Postage Stamps of the United Kingdom (1840–1905). London: Ewen's Colonial Stamp Market, 1905.