Philately of the War of the Pacific by Richard Abrams

Peru was a member of the Universal Postal Union, but Chile was not, and thus was not bound by international postal conventions. Many postal irregularities were committed, including the looting of post offices and impounding of postage stamps that were often used without payment. To correct these abuses, the Peruvian government hand-stamped oval, triangle and horseshoe overprints to insure that the Peruvian stamps were sold according to the regulations of the Universal Postal Union.