Well, I always thought it was the big bold letters on the side of the fuselage, but here's what wikipedia says:

"With the introduction of wide body aircraft, a new form of livery also came about called the billboard livery. With billboard liveries the actual name of the airline became the unique and main feature of an airlines fuselage livery. By abandoning the cheatline that had traditionally run the length of the aircraft along the windowline; billboard liveries dropped the window cheatline colors and extended the name of the airlines titles above as well as below the window line in a much larger script."

http://en.wikipedia.org/wiki/Aircraft_livery#Billboard

Now we know!! Says there were several carriers with *billboard* liveries -- which I didn't know!!  :o

Cheers!

Bruce

PS - Here's a video I made some time ago using the CS 707. There's a picture of me at my desk in 1963.

https://www.youtube.com/watch?v=TBtzhGwjqJ0&feature=em-upload_owner