World Stamp Expo '89 was held from November 17 to December 3 at Washington, DC.
It was held in conjunction with the 20th Universal Postal Congress.
Philippines issued three stamps for this event. These stamps-on-stamps issue picture the following:
P1 -- the first stamps of the Spanish era and Revolutionary period
P4 -- the first stamps of the US administration and Commonwealth period
P5.50 -- the first stamps of the Japanese occupation and Republic era
The P1 errs in design in that the Queen Isabella II stamp is perforated!