Auction #93 Worldwide Air Post stamps and postal history, Zeppelin Flight items, philatelic rarities of the World

1922, Provisional violet overprint ''Baku'' over inverted black re-valuating surcharge 50,000r on 3000r brown and blue, Baku postal cancellation, mostly VF, used stamp is a rarity, several experts' guarantees on reverse, Standard Cat. #26, $1,500 for an unused example, and not priced in used condition, in our opinion by mistake, because the only used stamp can be considered as real one, Est. $500-$600