MLA
Cooper, Samuel. Note from Samuel Cooper (Adjutant General's Office, Washington, D.C.) to R. H. Pruyn (Albany, New York), Acknowledging Receipt of Military Return - February 12, 1856. 1856. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 8, Folder 16. https://digital.library.emory.edu/purl/52908kpsg6-cor.
APA, 6th edition
Cooper, S. (1856). Note from Samuel Cooper (Adjutant General's Office, Washington, D.C.) to R. H. Pruyn (Albany, New York), acknowledging receipt of military return - February 12, 1856 (Box 8, Folder 16) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/52908kpsg6-cor.
Chicago
Cooper, Samuel. “Note from Samuel Cooper (Adjutant General's Office, Washington, D.C.) to R. H. Pruyn (Albany, New York), Acknowledging Receipt of Military Return - February 12, 1856.” Letters (correspondence). Civil War Collection, 1856. Box 8, Folder 16. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/52908kpsg6-cor.