The Post Office Department's "Philatelic Truck" was a mobile stamp collecting exhibit that ran between 1939 and 1941--not coincidentally during the presidency of famed stamp collector Franklin D. Roosevelt.  The truck traveled throughout the United States stopping at towns large and small with displays of famous stamps and offered stamps and other items for sale.

In January, 1940, the Post Office Department decided to add a cacheted envelope for sale only from the Philatelic Truck.  The Department created the cachet and it was added to existing 3c postal stationary envelopes in Washington, DC, then shipped out to the Truck while enroute.  The cacheted envelopes sold for 5c (3c postage, plus 2c for the envelope and cachet) and could be used for any regular mailing.  Buyers could mail it from the truck (which would transport pending mail to the nearest distribution center) or mail it from home at a later date.

This particular cover was mailed from San Diego, California on December 11, 1941.  This is an important and historical day, occurring 4 days after the Pearl Harbor attack, and being the day that Germany declared war on the United States, engulfing the US fully into World War 2.  Records indicate that San Diego was the final stop of the Philatelic Truck. It appears that a decision had been made to wind down the Truck's journey a couple of months earlier, but the Pearl Harbor attack and the opening of World War 2 confirmed the end of the program in order to focus on the War effort.  The San Diego stop of December 11-13, 1941 was the end of the Philatelic Truck's journey. 

This is a very scarce item!  Over the two years that the cachets were available, less than 20,000 cacheted envelopes were sold.  For comparison purposes, that's less than 10% of the 200,000 Graf Zeppelin stamps sold! A recent eBay search revealed no cacheted covers available.

I am listing this at auction at a low opening bid to let the market dictate the value of this item.  

Shipping will be by best price option that provides for tracking and insurance, depending on final sale price.