cover --, 1931, 30k-2r North Pole Zeppelins, imperforate (C26-C29), two complete sets, tied on four registered Zeppelin flight cards or covers, both sets with slightly different flight cachets, also incl. another Zeppelin flight cover with 35k and 2r values with yet a third different flight cachet, very fine, Scott retail $104.00 (as used stamps)