American definitives were overprinted
for use in Puerto Rico in 1899-1900.
On November 25, 1937 the United States issued a stamp depicting
La Fortaleza in San Juan as one of its Territorial Issues. Some
changes can be noted in the scene between 1937 and 2006.
One of the sentry boxes in Morro Castle in San Juan is featured
on the United States stamp issued on September 12, 1971
commemorating the 450th anniversary of the city of San Juan.