Update: I have received additional information that sheds light on this marker. The image is part of a large relief that features roughly 150 additional images that reference important events and people in American history. They include John Brown, William Lloyd Garrison, Booker T. Washington, the Atomic Bomb, and the Santa Maria, among others. The relief—executed by artist Laura Gardin Fraser—was completed in 1956 and dedicated in 1965. One of her most notable projects was the statue of Robert E. Lee and Stonewall Jackson that was located in Baltimore before its removal in 2017.