What makes this large oversize sketch sheet special is that it is the first set of ideas drawn for the Doubleday Books MARS series of Edgar Rice Burroughs novels. Also, notice that Frank's original idea for the cover depicts the scene where John Carter flies in the air attacking a thark. Ultimately, he rejected that idea and used the more standard heroic pose of John Carter and the Princess with the giant moon behind them. The rejected cover design was then used as an interior illustration.
The sketch sheet is simply a wonderful work of art in itself.