This is what I came up with:
"I will make this very clear. My one reason I am killing. If I don't kill I am likely too abandon my chance to collect my slaves for the afterlife. Even when I know you can't find me I am not safe walking if I am collect my slaves. When slaves you found saved dead in a car. He changed his mind and road form me. He walked by without hiding from me. Nothing was safe. He was hiding a knife. You even listened too my slave was calling for help."
This is a polyalphabetic substitution cipher, not a transposition cipher, which is why basic decryption programs that only have one letter substituted for a coded letter, are not compatible with this approach.
Another researcher and programmer pointed out that it has a 69%, one to one, accuracy rate. He said I could use this graphic he created to help illustrate the accuracy. In his email he explined.
I was playing around. I used your solution and made the assumption that each symbol would represent one letter in each quadrant. I used the first instance. Of course we know that Zodiac used multiple symbols to represent the same letter and also used symbols to represent multiple letters. By using the method I described, your solution proved 69% accurate.
Let me explain a little more in depth. If you go to the first quadrant (upper left), you will see that "R" occurs three times in the cipher. In your solution, "R" represents "I", "E", and "O" - in the first quadrant. In my test, it was only allowed to represent "I". I attached an image. When you look at the solution solved by quadrant and only using a single substitution method, it is impressive.
So far, this is by far the best solution I have seen. On an odd note, I found when you solve the entire cipher using your using the single substitution, it said something like, MAARYISASTINKINGWHORE," at the end. It had the name MIKE AVEY in the center. There is a military man from San Francisco named Mike Avey, but I haven't looked at it much because the rest of the cipher was gibberish using this method.
The first two lines only have one wrong letter. When you consider the frequency of the other correct letters for the solution is very compelling. Also the accuracy for the first 5 lines is about 85%, the first 7 lines about 79%, the first half, 70% accurate.This seems very compelling and highly unlikely that another solution could be fabricated that would have the same level of accuracy.
Here is a photo of the cipher and solution. The circled letters are the ones that correspond to other shared letters in the other words.
I started by dividing the cipher into four equal quadrants then making a frequency distribution noting which quadrant for each letter.
This is the frequency distribution for each character:
Notice that some of the coded letters are clustered in quadrants, while empty in others.
I took the most unbalanced coded letters. For example if there were four hits in Q1 and none in Q4.
And when these were circled on the cipher they also clustered together. This was my original.
I then noticed that in two of the clusters in Q1 (top left quad) on line one and four shared two characters that were inverted. It didn't take me long to hone in on "MA" and "AM", which was the first clue. And from the other Zodiac letters we know he starts with "This is the Zodiac speaking" But of course he wouldn't use that, so "i will MAke this very clear", was a logical start. At first I didn't know if it was "I will make this very easy, or hard." But "clear" became the most likely word after working out the rest of the cipher.
This is the frequency distribution for the solution.
The backbone of this solution is the first half, first nine lines, then line 14&15, "changed his mind", line 16 , "Hiding from me", line 16,17,18,19 "From me..." "was hiding a knife you even listened too my slave."
Then the gaps between had some strong words, however these were several other variations. On line 12 "saved" is very strong as well as "slaves" before that on line 11, for the subject, but "end" was solid for "dead" and could have been "ending his life a slave." But this wasn't the Zodiac's style of writing. He was always very clear and direct and his sentence didn't trail off with something confusing. And out of that variation, all there was was "end" and the rest wasn't solid.
"Without" on line 15 also had a variation with the same corresponding characters, but "hiding from." on the next line was still the verb. So, from entire sentence the "hiding from" was the only conclusive part, and the other word, "without" was the most probable considering the context. The other variation only had one extra letter, and didn't seem like Zodiac's style.
However, the other possible variations, would still use the same verb, and are very much in the same context and are not significant or indicate his identity. So, unless there is a variation with some compelling information, the minor ambiguous lines aren't enough to discredit this solution.
There were some very compelling occurrences that indicated most of this solution was an intentional method.
For example, on line four "Chance" and on line thirteen "Changed" both share the same characters for "HAN".
All of the "N" are the same character of the circle with the cross. The "^" has four "A", the "q" has 9 "L"'s, the backwards L had four "M's" backwards F has four N's.
The distribution in each quadrant is also noteworthy. The B has three S in Q4 and two A in Q1. which seems intentional that the letters of S and A were distributed evenly into separate quadrants. The "q" has five "L" in Q1.
What's more some of the more unusual combinations of coded letter and solution seem intentional. The solid triangle is an L in two places in Q1. The G is a C, V is a K, backwards L is an M in three places in Q1. Backwards F is an N in four places, one in each quad, and backwards F only occurs four times. The circle with a dot is an A all three times.
I didn't even notice any of these patterns and simply constructed the words that had the most corresponding similar letters.
I wonder if one of the victims had a knife that only the police know about?
The word "hiding a knife" had a strong hit.
There were some other variations with several of the other lines. For example, "car" didn't have any corresponding letters, and could have been "lot" or some other combination with the "In a car".
At fist I thought the last line was "stalling out hard" then I got "knife" and "ankle" on the 19th line, and thought the last line, line 20, could have been "falling out hard" as in a knife that was strapped to his ankle, and he was hiding sneaking away. However, "listened" seemed very strong. And once again "Slave" was just too strong in the end of the 19th line. I think he may have changed his mind in a few places.
I also think he was having schizophrenic episodes where his thinking was fragmented. There was a witness at the Stein cab killing. So, this does make sense. I also think he may have changed this to "calling out for help." It just seems like part of him wants to get caught and the other part is very deep into a compulsive disorder that he can't control.
Keep in mind that this cipher would have taken him a long time. It would have been a painstaking process to make it all work out to be neat and even. So that is why it may be slightly fragmented. Although, this solution in cleaner than the 408 solution that had a few extra letters here and there.
The spelling is also pretty good in this cipher. There was a "too" instead of "to", and the next "to" could have been an "at", as the only other solid "p" was a "t". I think he used the longer "too" to throw off the solution, but it is also possible it was a misspelling. Sometimes, people wonder why he spelled well in his ciphers or in some letters while not in others. He probably had a dictionary at home where he made this and his other quick notes were composed in a car. He didn't want to carry around any letters that could incriminate him and composed them on the spot and mailed them off. Most of them appear to be rushed like this. Some appear that he made most of it at home, then signed it right before mailing it. Regardless, I think people make too much out of his spelling or assume he could spell when he didn't unless he had a dictionary. Some words he probably intentionally misspelled to make it appear that he was intentionally misspelling words, but I don't' think his poor spelling was intended to prove he was unintelligent, as he didn't miss the chance to claim a superior intelligence and ridicule the authorities, and his spelling mistakes will help identify him.
Finally, the fact that there were such a high concentration of single coded letters in each quadrant, such as six "B" in Q4, five "q" in Q1, indicated that he substituted different letters for each coded character. If not there would have been very obvious words found a long time ago. There were a few places that had a slightly more accurate translation, but didn't fit the context as well, or didn't resonate the Zodiac's cadence and structure. He was pretty clear and straightforward.
Any suggestions, observations, or if you can find a more plausible variation using this approach, feel free to contribute!