Forsythia, 1966. “The design of ‘Forsythia’ is made from the letters of the name of the flowering shrub and their equivalents in the Morse Code. The text is part of the design.”--Mary Ellen Solt
There are 75 unpublished items by Mary Ellen Solt at Indiana University's Lily Library. The earliest of these date from 1960. Present are letters from Robert Creeley, Denise Levertov, George Oppen, John Thirwall, and Louis Zukofsky. Also present is a notebook prepared and titled by Solt "Index to Robert Creeley Ms Material--Lilly Library"; two Christmas cards from Florence (Mrs. William Carlos Williams); a holiday card and two color transparencies from Paul Williams relating to his father, William Carlos Williams; and a file of eighteen photographs and negatives of William Carlos Williams and family members.
John ("Jack") Thirwall was supposed to be an official biographer of Williams. His book never came out, and I've for years tried to find out what happened to Thirwall and/or his project. No luck so far. I'm guessing that the Thirwall materials here are letters from him to Solt asking about WCW.
Kenny Goldsmith's UBUweb has her entire Flowers in Concrete (1966) here. UBU also has an essay by Solt on the origins of concrete poetry in Brazil.