Coamo Provisional. 1898, 5c black, type II, position 10, ample to large margins showing complete frame lines, good impression, used on envelope to Presidio, California (Potts correspondence), tied by « Military Sta.1/Porto Rico.Wash. D.C. » duplex cancel, repeated adjacent, backstamped with New York, San Francisco and Presidio ds; one of only two Coamo covers known bearing the franking tied by a postmark, a key-gem of Puerto Rican philately; cert. PF (1992); Scott $27’500; ex Gallagher and Acevedo.
Note: Only 500 examples of this stamp were printed and, according to the Siegel Census, just ten covers have survived, of which three were sent to San Francisco, sent by Captain Potts to his wife, these being the only covers extant sent abroad bearing a Coamo Provisional. As the stamp was only valid internally, it paid the portion of the journey from Coamo to Ponce, and this mail should have required 2c postage for onward transmission, but it was nevertheless accepted without additional franking by the military postal clerk.