This is a large cover that was flown on the First Flight of the San Francisco to Hong Kong Pan American Airlines and the Royal Dutch Air Line (KLM) Flight in 1937.  This cover stopped and was franked at four different countries (USA, Netherlands, Straits Settlements (Malaya) and Brazil)  The stamps are all tied by the correct cancellation for the flight.  There are many marking on the cover including the flight cachet, and markings from the two airlines.  The cover has labels to direct it to the next location.

San Francisco to Hong Kong (KLM), 1937

This is also a Boy Scout Jamboree related item for one of the stamps from the Netherlands is a special issue for the Fifth World Jamboree to be held in Holland that year.