Registered airmail cover franked with the 1939 'Chaco Boundary Peace Conference' issue complete AIRMAIL set of nine on front plus 1939 11p blue green 'New York World's Fair' issue on reverse (SG 520/528 & 531) all tied by ASUNCION cds's dated 28.3.1940 with printed registration label on reverse. Addressed to USA with transit & arrival marks on front & reverse. Very attractive.