Series V. 2011 Addition contains photocopies of letters from Eugene and Carlotta O'Neill to Kenneth Macgowan, along with four autograph manuscript letters, signed and dated 1951, from Carlotta.
Series VI. 2015 Addition contains Proofs of Mourning Becomes Electra, dated 1931, and a letter from Eugene O'Neill to the editor of the Yale Daily News, dated 1921.