MLA

Speck, William A. (William Alfred), 1864-1928. ALS : Weimar, to Johann Wolfgang von Goethe. Box 12, folder 475. 1821 Jun 28. https://collections.library.yale.edu/catalog/2005379.

APA, 6th edition

Speck, William A. (William Alfred), 1864-1928. (["Box 12, folder 475"])(1821 Jun 28)[ALS : Weimar, to Johann Wolfgang von Goethe]. https://collections.library.yale.edu/catalog/2005379.