From the Annals of Aviation – In 1926, the first commercial air mail service (known as CAM-3) to and from Texas was begun. The initial route was between Dallas and Chicago. The initial CAM-3 service was awarded to National Air Transport, Inc. which had been founded in 1925 by Clement M. Keys. CAM-3 air service was inaugurated with both north and south flights between Chicago and Dallas with interim stops in Moline Illinois, Saint Joseph & Kansas City Missouri, Wichita Kansas, Oklahoma City Oklahoma and Fort Worth. NAT used Curtiss Carrier Pigeon bi-planes with Curtiss engines for this initial CAM-3 service. The Carrier Pigeon bi-planes were built by the Curtiss Aeroplane Company founded by Glenn Curtiss and now controlled by Clement Keys. Postal mail covers carried on the inaugural flight between each point of landing and take-off are collectible and are known as CAM-3 covers.
Photo from http://www.aerodacious.com.