Discovery of phage λ (lambda phage). According to estherlederberg.com, only about 100 people received the first issue of Microbial Genetics Bulletin, a typed and mimeographed publication. Digital facsimile from estherlederberg.com at this link.
Esther Lederberg's discovery became much better known in a paper that she co-authored with her husband, Joshua Lederberg: "Genetic studies of lysogenicity in Escherichia coli," Genetics 38 (1953) 51-64. That paper was enhanced with photographs of the action of the phage on sensitive lysogenetic and resistant strains of E. coli. Digital facsimile of the 1953 paper from PubMedCentral at this link.
(Thanks to Juan Weiss for this reference and its interpretation.)