I believe you have Puerto Rico overprint issued in 1900. It is listed in the Scott Specialized Catalog of US Stamps in the "Puerto Rico" section, as Scott #215 (black overprint "Puerto Rico" used on U.S. 1-cent #279 in yellow green).
My US Specialized catalog is quite old, but a VF used copy had a 2002 price of $1.40 at that time. Someone else may want to chime in and confirm if there has been any movement on the value of that stamp in recent years.
The stamp is one of a series of stamps overprinted for use in the various Spanish possessions which became US possessions following the Spanish-American war.
Stamps were overprinted for Cuba, the Philippines, and Puerto Rico.
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