It has been noted that if the code were fake why bother to correct the sixth line symbol with a reversed K, after all it would have made little difference to his objective and if it were genuine why correct it anyway, after all it is not like his previous 408 cipher was free of numerous spelling errors and if the Zodiac had intended to write the word 'animal' as he had done previously and accidentally inserted 'anamal' , why not just leave it, he tried to confuse people anyway, unless of course what he was writing was of the utmost importance it needed to be corrected like a date, time, name or more importantly, something to do with himself.
It is is a fair assumption to conclude based on the numerous correspondence he engaged in with authorities, that was considered genuine, that the presumed Zodiac was an egotist, who craved attention through the media outlets and took letter writing to a new art form, driven by a determination to usurp all previous serial killers by shrouding himself in even more notoriety than those that had gone before him, including Jack the Ripper, who terrorized the Whitechapel district of London some eighty years ago and for the large part probably achieved all but the last one.
So what of it, well as an example could you appreciate the importance of his correction if it meant that 'Zodiac' was not to become 'Todiac', an error with a personal attachment that would have infringed upon his already inflated self importance, similarly a date, not unlike the one scrawled on the car door at Lake Berryessa, a key integral part of the cipher that he felt compelled to rectify.
An overriding thought is that whoever penned the 340 cipher created a genuine mystery, based on the reasons above as well as the proverbial self satisfaction 'pat on the back' factor he would have gleaned from watching authorities fail to decipher a real code as opposed to a fake one, which would have been much shorter lived and would have failed to satisfy the risk element he so clearly craved.
It has also been suggested that he deliberately faked the correction to conceal the fact that the 340 was not authentic, which would have propelled him to even more heights of deviousness, but again the lack of longevity of such a master plan would not have satiated his desire during the cold winter nights ahead.
To understand a killer is to be one and trying to imagine his reasoning from the standpoint of a normal individual makes the task of quantifying his thoughts an almost impossible task, as is trying to make sense of the unsensible, so despite everything that is written above, unless I am bordering on insanity, it all makes perfect sense to me, but unfortunately probably makes little sense to the Zodiac.