1938 Sc 497-500 First Flight Moscow-Portland over North Pole Scott 636-639

1938. First Flight Moscow-Portland over North Pole.
Catalog numbers: Scott 636-639, Standard Collection 497-500, Michel 595-598.
Issued on: April-June, 1938.
Perforation: line12¼.
Printing: Collotype.
Colors: Multicolor.
Face value: 10 Russian kopek; 20 Russian kopek; 40 Russian kopek; 50 Russian kopek.
Quantity: 2,900,000; 1,900,000; 1,100,000; 1,000,000.