Sandburg's addenum, "New American Songbag," was printed in hardcover in 1950 but quickly came out in a spiral bound paperback. It is only 110 pp. Other paperbacks came out in the 60s and later. I just ordered one, so looked up what editions are available.
Amos, I agree with your interpretation. It is the one Kendall Morse used, and the only one that makes sense to me.