MLA

Koch, Frederick R. (Frederick Robinson). Early version titled "Siegfrieds Tod": libretto, with music quotation. Box 59, folder 1355. 1848 Nov 12-28. https://collections.library.yale.edu/catalog/2004964.

APA, 6th edition

Koch, Frederick R. (Frederick Robinson). (["Box 59, folder 1355"])(1848 Nov 12-28)[Early version titled "Siegfrieds Tod": libretto, with music quotation]. https://collections.library.yale.edu/catalog/2004964.