This is a partial answer to my question about Jitsuro Nagura, if anyone else happens to be interested. His Japanese name, in Chinese characters, is 奈倉実郎, which is similar except for the first character to that suggested by Willie above.
The Japan Academy reports that they have only two pieces of information about JN. First, he was born prior to 1918 and has a son, Riishi, who was born in 1938. Second, that son was a professor of electrical engineering at Himeji Institute of Technology and Kanagawa Institute of Technology. He was also an employee of the Japan Electronics Company, NEC. An anonymous correspondent at Kanagawa shared: "I am regret to say but Professor Nagura is no longer work for us." This has the ring of truth and JN's son is no doubt retired.
The paucity of information is amazing to me, and I intend to pursue it further, time allowing. Meanwhile, if anyone comes across anything more, I would be very interested.
Update (2/12): Nagura was an "early graduate" of Tokyo University (math major). Since Nagura's paper on primes was published in 1952, it is difficult to know whether he attended Tokyo University under its present name or the pre-1947 designation, Tokyo Imperial University.
Update (6/13): A paper with basic biographical information and a photo of Nagura as a young man in BSHM.
Update (8/13): Teiji Takagi, whose doctoral advisor was David Hilbert, was actively involved in cryptography during WW2. According to Nagura's son, Takagi was a professor of Nagura's at Tokyo Imperial university (possibly his advisor). It seems quite possible that Nagura also helped in the cryptography effort but it would be hard to verify.