Overprint (English Wikipedia)

Ceresa, Dr. R.J. (May 1986). The Postage Stamps of Russia, 1917-23. Vol. 2. Ukraine. Part 9/13. The Trident Issues of Kyiv Types I, II and II. Russian Philatelic Desk Top Publications, Felpham, Great Britain. p. 170. ASIN B0007BFS90.
Ceresa, Dr. R.J. (August 1978). The Postage Stamps of Russia, 1917-23. Vol. 1. Armenia. Part 3. The Large sized Framed HP Monogram Overprints. Russian Philatelic Desk Top Publications, Felpham, Great Britain. pp. 62–63. ASIN B0007BFS90.

archive.org (Global: 6^th place; English: 6^th place)

Klug, Janet (2008). Guide to Stamp Collecting. New York: HarperCollins. p. 45. ISBN 9780061341397.

"Bills Stamped with Hawaii". Infoplease. 1944-10-21. Retrieved 3 September 2011.

Schenk, Robert (2011). "A Case of Inflation". Saint Joseph's College Economics Dept. Retrieved 3 September 2011.

Klug, Janet (3 June 2002). "Surcharges and overprints make a difference". Linn's. Retrieved 3 September 2011.
Miller, Rick (2011). "Lines of stamp classifications have blurred". Linn's World Stamp Almanac. Archived from the original on 25 June 2008. Retrieved 3 September 2011.

Gallegos, GF (2002). "El Salvador Handbook - Chapter 3: 1874 - 'Contrasello' Issue". Archived from the original on 16 June 2019. Retrieved 16 June 2019.

USPS (2010). "Domestic Mail Manual" (PDF). United States Postal Service. Retrieved 3 September 2011.

Gallegos, GF (2002). "El Salvador Handbook - Chapter 3: 1874 - 'Contrasello' Issue". Archived from the original on 16 June 2019. Retrieved 16 June 2019.
Miller, Rick (2011). "Lines of stamp classifications have blurred". Linn's World Stamp Almanac. Archived from the original on 25 June 2008. Retrieved 3 September 2011.