Cite

"M.S. of Peter Pan. The only one in existence so far as I know"

MLA

Barrie, J. M. (James Matthew), 1860-1937. "M.S. of Peter Pan. The only one in existence so far as I know". Box 44, folder 945. 1928 July 18. https://collections.library.yale.edu/catalog/17476337.

APA, 6th edition

Barrie, J. M. (James Matthew), 1860-1937. (["Box 44, folder 945"])(1928 July 18)["M.S. of Peter Pan. The only one in existence so far as I know"]. https://collections.library.yale.edu/catalog/17476337.