cover Stratosphere Balloon Covers, 1934-35, record breaking altitude 'Explorer I'and 'Explorer II' flight covers, balloons launched jointly by the National Geographic Society and the U.S. Army, only 500 covers of former were flown; also incl. two 'Explorer II' bookmarks (made from the balloon's fabric) and a third still mounted on the original presentation folder, the covers are fine-very fine, AAMC $100.00+