(C1) 1919 Caribou 3c brown overprinted "''FIRST / TRANS- / ATLANTIC / AIR POST / April, 1919", the so-called "Hawker", lightly hinged, exceptionally well centered, signed on back by the postmaster "J.A.R." (J.A.Robinson), also Giulio Bolaffi and Alberto Diena, with 2023 Greene Foundation certificate. A fresh and v.f. example of this Air Post rarity (the stamp was overprinted for use on mail carried by Harry Hawker and Kenneth Mackenzie Grieve on their proposed non-stop flight across the Atlantic in a Sopwith biplane. The aircraft came down in the sea but the men were rescued and the plane salvaged. Only 200 stamps were overprinted, of which 95 were used on letters, 11 were given as presentation copies, 76 were sold in aid of the Marine Disasters Fund, the remaining 18 were apparently damaged and destroyed), cat. $25,000