MLA

Gellert, Lawrence, 1898-1979. American negro folk music --an outline. Box 2 | Folder 32. undated. https://collections.library.yale.edu/catalog/10589203.

APA, 6th edition

Gellert, Lawrence, 1898-1979. (["Box 2 | Folder 32"])(undated)[American negro folk music --an outline]. https://collections.library.yale.edu/catalog/10589203.