http://www.aerodacious.com/ccCAM027.HTM

CAM-27: CAM-27 was awarded to Thompson Aeronautical Corporation. TAC was founded in early 1928 and became the successful bidder for the mail route from Bay City MI to Chicago IL. Postal mail covers carried on the inaugural flight between each point of landing and take-off are collectible and are known as CAM-27 covers.

First flight to Ann Arbor, July 17, 1928

http://www.mlive.com/news/annarbornews/index.ssf?/base/news-33/1246776048204790.xml&coll=2

http://www.flickr.com/photos/70251312@N00/8408625167/in/photostream