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

1991, Endangered Species, 1000r multicolored, complete set of six imperforated proofs with control angles at corners, printed on thick glazed paper, cut from composite proof sheetlet of six, each one with black overprint ''Prueba'' (Proof) on reverse, no gum as produced, NH, VF and very rare, reportedly only 45 sets were sold, the only uncut sheet retained in the Mexican Postal Museum, Est. $350-$500