MLA
Mosby, John Singleton, 1833-1916. Letter from John Singleton Mosby to General Marcus J. Wright commenting on some statements written by Beverly H. Robertson in the "Century" in answer to an article by Mosby (photocopy). between 1870 and 1890. Civil War Collection. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/4472v6wxrh-cor.
APA, 6th edition
Mosby, John Singleton, 1833-1916. (between 1870 and 1890)[Letter from John Singleton Mosby to General Marcus J. Wright commenting on some statements written by Beverly H. Robertson in the "Century" in answer to an article by Mosby (photocopy)]. Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/4472v6wxrh-cor.
Chicago
Mosby, John Singleton, 1833-1916, Letter from John Singleton Mosby to General Marcus J. Wright commenting on some statements written by Beverly H. Robertson in the "Century" in answer to an article by Mosby (photocopy), between 1870 and 1890, Civil War Collection, Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/4472v6wxrh-cor.