MLA

Speck, William A. (William Alfred), 1864-1928. ALS : Weimar, to Friedrich Ludwig Schröder. Box 9, folder 297. 1791 May 24. https://collections.library.yale.edu/catalog/2005333.

APA, 6th edition

Speck, William A. (William Alfred), 1864-1928. (["Box 9, folder 297"])(1791 May 24)[ALS : Weimar, to Friedrich Ludwig Schröder]. https://collections.library.yale.edu/catalog/2005333.