Two flown covers carried on experimental aircraft, both with stamped cachets filled out with information on the flights, signed by the respective pilots. Includes: one cover flown on board Lockheed Jet VTOL XV-4A 'Hummingbird' at Dobbins AFB for hover tests on June 24, 1963; and one cover flown on board X-22A V/STOL at Niagara Falls for a 25 minute flight at 4000 feet on August 24, 1967. In overall fine condition, with subtle toning to the 'Hummingbird' cover.
