J.R. Oppenheimer and Julian Schwinger. "On the Interaction of Mesotrons and Ncueli." In: the Physical Review, volume 60, 15 July 1951. Pp 150-152. Offered in the original green-wrapper issue for 15 July, comprising pp 67-174. Very good copy. $150 [Referenced in Robert Serber's "Partcile Physics in the 193o's" in Laurie Brown's The Birth of Particle Physics.