Air Mail Route 66, a new route to Boston inaugurated on May 1, 1945, enabled United Air Lines to route transcontinental planes into Boston from Cleveland, Ohio, via Hartford, Connecticut. Unofficial cachets sponsored by United Air Lines were applied at Boston (showing the Bunker Hill Monument) and Hartford (showing the Charter Oak). 

This cover was carried on the inaugural Air Mail Route 66 flight from Boston, Massachusetts to Cleveland, Ohio and is listed in the Contract Air Mail Flights (CAM) Section of The American Air Mail Catalogue as 66W1.