MLA
Stephenson, J. H. Letter from Stephenson (Boston, Massachusetts) to John A. Andrew, Governor of Massachusetts. J. H. Stephenson Was a Recruiting Officer for the Union Army - April 1, 1864. 1864. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 20, Folder 26. https://digital.library.emory.edu/purl/4644qrfkch-cor.
APA, 6th edition
Stephenson, J. H. (1864). Letter from Stephenson (Boston, Massachusetts) to John A. Andrew, Governor of Massachusetts. J. H. Stephenson was a recruiting officer for the Union army - April 1, 1864 (Box 20, Folder 26) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/4644qrfkch-cor.
Chicago
Stephenson, J. H. “Letter from Stephenson (Boston, Massachusetts) to John A. Andrew, Governor of Massachusetts. J. H. Stephenson Was a Recruiting Officer for the Union Army - April 1, 1864.” Letters (correspondence). Civil War Collection, 1864. Box 20, Folder 26. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/4644qrfkch-cor.