Canal Zone, Post Office Seal, 1917, dark blue, three singles, affixed around 1920 cover from Philadelphia to New York and forwarded Cristobal and Balboa Heights, in combination with U.S. Post Office Seal 1917 black (OX18) at top, franked with U.S. 2¢ carmine tied by Philadelphia, Pa. machine postmark; few cover tears, Very Fine, a rare post office seal combination. Scott No. OX3  Estimate $400 - 600. $0 (Image)