Cracking the Japanese Water Transport Code

From Code Girls, by Liza Mundy (Hachette, 2017), Kindle pp. 241-242:

Ambon. Canton. Davao. Haiphong. Hankow. Kiska. Kobe. Kuching. Kupang. Osaka. Palembang. Rabaul. Saigon. Takao. Wewak. Dot Braden until a few months earlier had never heard of most of these places. Now they ruled her life. They kept her running from the big table where she worked, over to the overlapper’s console, then back again to her spot at the big table. These were the names of places, somewhere in Asia or the South Pacific, likely to be mentioned toward the beginning of messages coded in 2468, the main Japanese water-transport code, or one of the other, smaller transport codes.

Or rather, they were some of the places. Transport code 2468 was massive; 2468 was everywhere; 2468 dominated the Pacific Ocean. Anything anybody needed was sent by water. Water was how the rice was transported, and the soldiers, and the spare airplane parts. To move the goods the Japanese Army needed, the marus were always sailing. Always leaving and arriving. A maru [丸] could be a tanker, a freighter, a cargo ship, a barge, a cable layer, a motor transport. [Japanese Navy ship names never use maru.] They plied between Hiroshima, Yokohama, Wewak, Saipan, Tokyo, Manila, the Truk Lagoon. Exotic places. It was not necessary for Dot to know how to pronounce the cities and ports, but it was helpful to know the four-digit code groups that stood for them. Code system 2468 commanded Dot’s attention, controlled Dot’s movements. It filled her brain.

A job more unlike teaching Virginia schoolchildren would be hard to imagine. No longer was Dot Braden standing at a chalkboard, explaining physics formulas to eye-rolling teenagers, or ordering senior girls to march and salute. Instead, she was sitting head down at a table puzzling over words she had never heard before she came to Arlington Hall. “Sono.” “Indicator.” “Discriminant.” “GAT.” The sono ['that, aforementioned'] was the number appended to messages that had been divided into parts before being transmitted. Sono #1 was the first part, Sono #2 was the second part, and so on. The discriminant was the number that identified the system—for instance, 2468. The indicator was the tiny clue that told you what book to look in. GAT stood for “group as transmitted”: the code group plus the cipher. The GATs were what you saw when you looked at the message for the first time.

Dot, of course, was not to utter any of these words outside the high wire double fences of the Arlington Hall compound. People were warned never to use, outside the building, the words they used inside it. “This material is extremely secret and must be treated with the utmost care,” one training document said. “Some of the words which you will consider elementary have been used only in this code, eg KAIBOTSU SU ‘to sink a ship’ [海没す 'sea-reject do'?]. If you should mention this word to any one connected with the Axis or in some way succeed in letting it get into improper hands, this one fact alone would betray to the Japanese that we are reading their most recent transport code.”