MLA

United States. Army. Military Division of the Missouri, Williams, Robert, fl. 1876. Dispatch [duplicate of autograph letter signed] 1876 July 16, Omaha [to] General Crook, Fort Fetterman. 1876 July 16. https://collections.library.yale.edu/catalog/2064421.

APA, 6th edition

United States. Army. Military Division of the Missouri, & Williams, Robert, fl. 1876. (1876 July 16)[Dispatch [duplicate of autograph letter signed] 1876 July 16, Omaha [to] General Crook, Fort Fetterman]. https://collections.library.yale.edu/catalog/2064421.