US-2388538 traveled from Washington D.C. to Hawaii in 3 days. It was from another exhibition "Fire & Ice: Hindenburg and Titanic" at the National Postal Museum in Washington D.C., showing the airship Hindenburg.
Both Hindenburg and Titanic attempted to fill the demands for the mass travel and communication demands at the early 1900's. Both operated as the world's largest mobile post offices promising the fastest mail service in its day. Ironically, both ended in disasters. The exhibition shows rare artifacts including a mail sent from Titanic and burnt mail salvaged from Hindenburg.
The exhibition opened on March 22, 2012 and runs through January 6, 2014. For those who can't attend in person, please visit the online gallery on the web.