Stinnett's book is dense at best, because it discusses many different codes, including the Japanese diplomatic Purple and J-series codes, plus the Kaigun Ango or Naval Codes, of which there were several.
By 1940 all the Japanese codes had been broken, and were routinely read in almost real time. Our guys fully understood not only the codes themselves, but the ways in which they were periodically changed. At most, it was a matter of a couple of days to read Japanese messages.
Amongst the Kaigun Ango or Naval Codes was the Ship Movement Code, the SM code, "a system for reporting arrival and departures of naval vessels from Japanese ports and anchorages..." p.72
Stinnett, pg 72-73: "Japan's SM code first disclosed the Hawaii plans early in November [1941].
"This code proved a gold mine for American cryptographers.
"It provided 210 movement reports of Japanese warships between November 2 and December 4.
"Every class of warship is represented, from mine layers to the carriers of the First Air Fleet.
"None of the SM dispatches was ever examined by Pearl Harbor investigators."
Stinnett says nothing about your "AN Code."
I suspect a typo, or perhaps like me, it is just too easy to get lost in Stinnett's discussion of all the different codes, who was intercepting what, which ones were decoded & read, reported on, etc., etc. ;-)
Stinnet does talk about a lot of codes. AN-1 and its group additives were an administrative code like Purple only it was for the IJN. On August 1st the Japanese added 50,000 new additive to the code book that made it unreadable. By Novemeber of 1941 only 3,800 code groups had been identified which still meant the code was unreadable.
Please understand that I not saying that Stinnet is wrong; I just pointed out that I didn’t see any mention of this pretty critical code that was used by the Japanese Navy. It is entirely possible that I missed it. I think Stinnet makes some compelling points in his book. He definitely give food for thought on the entire scenario surrounding the eventual attack on Pearl.