This primary source comes from the Collection FL: Frank W. Legg Photographic Collection of Portraits of Nineteenth-Century Notables.
Full Citation: Photograph FL-FL-6; Mark Twain (Samuel Clemens); ca. 1862 - 1884; Series: Portraits, 1862 - 1884; Collection FL: Frank W. Legg Photographic Collection of Portraits of Nineteenth-Century Notables; National Archives at College Park, College Park, MD. [Online Version, https://docsteach.org/documents/document/mark-twain, May 4, 2024]