cover British Guiana, 1921, Experimental Air Service between Cayenne-St. Laurent du Maroni, two flight covers, each franked with the special stamp (75c Black on grayish, Yvert 6A), with additional 25c frankings, special stamps tied by c.d.s. and straight line "Avion" h.s., very fine, AAMC $1,000-$2,000