JN-25 traffic increased immensely with the outbreak of naval warfare at the end of 1941 and provided the cryptographic "depth" needed to succeed in substantially breaking the existing and subsequent versions of JN-25. The American effort was directed from Washington, D.C. by the U.S. Navy's signals intelligence command, OP-20-G; at Pearl Harbor it was centered at the Navy's Combat Intelligence Unit (Station HYPO, also known as COM 14), led by Commander Joseph Rochefort. However in 1942 not every cryptogram was decoded, as Japanese traffic was too heavy for the undermanned Combat Intelligence Unit. With the assistance of "Station CAST" (also known as COM 16, jointly commanded by Lts Rudolph Fabian and John Lietwiler) in the Philippines, and the British Far East Combined Bureau in Singapore, and using a punched card tabulating machine manufactured by International Business Machines, a successful attack was mounted against the 4 December 1941 edition (JN25b). Together they made considerable progress by early 1942. "Cribs" exploited common formalities in Japanese messages, such as "I have the honor to inform your excellency" (see known plaintext attack). Later versions of JN-25 were introduced: JN-25c from 28 May 1942, deferred from 1 April then 1 May; providing details of the attacks on Midway and Port Moresby. JN-25d was introduced from 1 April 1943, and while the additive had been changed, large portions had been recovered two weeks later, which provided details of Yamamoto's plans that were used in Operation Vengeance, the shooting-down of his plane. This was a naval code used by merchant ships (commonly known as the ""maru" code"), broken in May 1940. 28 May 1941, when the whale factory ship "Nisshin Maru No. 2 (1937)" visited San Francisco, U.S. Customs Service Agent George Muller and Commander R. P. McCullough of the U.S. Navy's 12th Naval District (responsible for the area) boarded her and seized her codebooks, without informing Office of Naval Intelligence (ONI). Copies were made, clumsily, and the originals returned. The Japanese quickly realized JN-39 was compromised, and replaced it with JN-40. JN-39 was replaced by JN-40, which was originally believed to be a code super-enciphered with a numerical additive in the same way as JN-25.