Worldwide Air Post stamps and postal history, Zeppelin Flight items, philatelic rarities of the World. Part 3

Soviet Union Stamps of 1924-40

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Start price: $500

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1933, Baku Commissars, trial color perforated proof of 5k in brownish gray, printed on paper with watermark Borders and Rosettes, perforation L13¾, gummed, VF, Est. $750-$1,000 , Sc. #520