If ADM Nimitz needed any more evidence confirming the magnitude of the Japanese effort, he got it on April 24 when one of CDR Rochefort’s newest codebreakers, Japanese linguist LT Joseph Finnegan discovered a reference to a new organization, the “MO” covering force!”
Unraveling a radio message from Japanese VADM Shigeyoshi Inoue to his invasion force, LT Finnegan found that VADM Inoue had assigned a sequence of special call signs and designators for the approaching Coral Sea campaign. It listed no fewer than seven separate forces: MO fleet, MO occupation force (listed twice), MO attack force, RZP occupation force, RXB occupation force, RY occupation force.
The big question concerned MO. What did it designate? No one had ever heard of it. Intelligence officers were mystified. Why did MO, whatever it was merit no fewer than four occupation forces? Was this another term for Port Moresby? If so, what was RZP? Or did MO designate an altogether different target? The surfacing of MO ignited a new debate. The Imperials Japanese Navy’s overall strategy was by no means considered certain.
To settle the MO matter, CDR Rochefort turned the investigation of this two-kana diagraph over to his two top linguists, Maj Alva B. (Red) Lasswell, and LT Joseph Finnegan. Proficient in Japanese and adept at codebreaking they both had some skill at cryptanalysis. CDR Rochefort got the best out of both of them.
“A LT Finnegan hunch checked out by Maj Lasswell’s siege tactics made a firm foundation on which to build,” intelligence officer, Wilfred “Jasper” Holmes observed. That turned out to be the case with MO. LT Finnegan insisted it was Port Moresby.
But CDR Rochefort was skeptical, or at least pretended to be. He often played the role of the doubter to force his analysts to de more digging and support their contentions. “When Maj Lasswell backed up LT Finnegan, CDR Rochefort was convinced and so was Commander-In-Chief Pacific,” Holmes said.
On April 29, five days later, the digraph MO first appeared in intercepted Imperials Japanese Navy radio message traffic, all doubts vanished. CDR Rochefort’s analysis decrypted and translated Operation Order No. 1, in which ADM Yamamoto communicate to VADM Inoue, Fifth Carrier Division, Fifth Cruiser Division and his Eleventh Air Force to state the mission:
“The objective of the MO will be first, to restrict the enemy fleet’s movements and will be accomplished by means of attacks on outlying units and various areas along the north coast of Australia. The Imperials Navy will operate to its utmost until this is accomplished.”
Source: Elliot Carlson, “Joe Rochefort’s War”