From Ultra in the Pacific: How Breaking Japanese Codes and Ciphers Affected Naval Operations Against Japan, 1941-45 (The Secret War), by John Winton (Sapere Books, 2022), Kindle pp. 294-296:
ULTRA betrayed not only convoys but single blockade runners. The fate of I-29 was a perfect ULTRA coup. I-29, named Matsu, was the submarine which rendezvoused with a German U-boat off Madagascar in April, 1943, embarked Subhas Chandra Bose, the leader of the movement for Indian independence and self-styled C-in-C of the Indian National Army, and took him to Penang. I-29 (Cdr T. Kinashi) left Penang, bound for Europe, early in November, 1943, and sailed from Lorient, bound for Japan, on 16 April, 1944. Among the passengers were four German technicians and thirteen Japanese Army, Navy and civilian personnel. The cargo included German anti-submarine counter-measure equipment, acoustic and magnetic torpedoes, radar apparatus, plans for the latest high-submerged-speed submarines, and influenza virus.
I-29’s passage was traced through intercepted signals from Berlin, and a Singapore broadcast in diplomatic code, addressed to I-29 only on 3 July, indicated its presence in the Indian Ocean. An ULTRA from Anderson on it July read: ‘Friendly sub [identified as probably I-29] scheduled to pass through Sunda Strait on morning of 12 July, and arrive at eastern entrance to Singapore at 1200 on 14th.’ It was later confirmed by ULTRA that I-29 had indeed arrived that day.
On 17 July, 1944, a decrypted message from Berlin to Tokyo listed I-29’s cargo in detail: five ‘special weapons’, various radar apparatus, 20 Enigma coding machines, ordnance parts, rocket-type launching apparatus, bomb sight plans, pressure cabin parts and plans, parts of a British Mosquito plane, and atabrine ampoules and tablets. Two days later, in a decrypted message, Berlin congratulated Tokyo: ‘It is indeed gratifying to learn that the MATSU has arrived safely at Singapore with her passengers and cargo. We pray for her safe voyage to Japan.’
But on 20 July Kinashi broadcast a fatal signal giving full details of his route to Japan: leaving Singapore at 1500 on the 22nd, arriving Kure at 1000 on 30 July, and giving his noon position for the 26th as the Balintang Channel [between Formosa and Luzon]. CincPac’s Bulletin for 24 July read: ‘I-29 recently arrived Singapore from Europe carrying samples and plans of many recent German developments in fields of radar, communications, gunnery, aeronautics and medicine. Left Singapore 22 July en route Kure. Believe very important cargo very likely still aboard. Will pass through posit 15 N., 117 E., at 251400 and through Balintang Channel at 261200, speed 17 arriving western channel of Bungo Channel at 291000.’
On 25 July I-29 signalled that a surfaced enemy submarine had been sighted (possibly the ‘cover story’) and gave the position, about 300 miles west of Manila. On the 26th Sawfish (Cdr A. B. Banister, leader of ‘Banister’s Beagles’) signalled: ‘He did not pass. At 0755Z [1655 local time] in posit 20-12 N., 121-55 E. [Balintang Channel] put three fish into Nip sub which disintegrated in a cloud of smoke and fire.’
On 7 August a mournful Tokyo broadcast to Berlin was intercepted: ‘All her passengers had proceeded to Tokyo from Singapore by plane, but her cargo had been left aboard. Though it is indeed regrettable, we can no longer hope for her safety. Despite the fact that we received, through your great efforts and the understanding cooperation of the Germans, many articles which were to strengthen the nation’s capacity to prosecute the war, our inability to utilize them owing to the loss of the ill-fated ship is truly unfortunate and will have a great effect throughout the Imperial Army and Navy.’
I’m pleased to see that Cdr. T. Kinashi’s name is spelled consistently in this book, and that this remarkable naval officer has a detailed article in English Wikipedia (linked above). Several other Japanese names are handled quite sloppily. For instance, the IJN destroyer Kuroshio (‘Black Current’) is consistently misspelled Kurishoyo six times on pp. 276-278, and Lt. Gen. Kuribayashi, the Japanese Army commander on Iwo Jima, is transliterated correctly on p. 304, then misspelled as Kuribayasha twice on p. 306. (The Hawaiian place name Wahiawa is also misspelled as Wahaiwa on p. 158.)