An interesting article on Jos Kirps's Popular Science and Technology Blog, indicated some extremely close connections, that suggested the 'Halloween' card mailed to the San Francisco Chronicle on October 27th 1970 may have been the Zodiac Killer's biggest clue to date.
He observed that on the 'Halloween' card the Zodiac Killer had added the phrases By Fire, By Gun, By Knife and By Rope in four quadrants around the intersecting Paradice and Slaves. However, if the 340 Cipher was also separated into four quadrants, the word BY could be found in all four quadrants also. If that was not curious enough, he noticed the possibility that the word PARADICE ran down the center of the 340 Cipher, and the word SLAVES possibly across its midsection.
So in the following section we shall use this as the starting point and attempt to find some more connections to the 'Halloween' card and a few other letters.
So let us add the words FIRE, GUN, KNIFE and ROPE underneath the words BY on the 340 Cipher.
On the 'Halloween' card the Zodiac Killer places FIRE, GUN and ROPE underneath the letter B of BY. But the word KNIFE is placed under the letter Y of BY. (as the By runs vertically). In the diagram below Fire, Gun, Knife and Rope have been placed one column to the side so as not to obscure the original symbols.
If you take a look at the word KNIFE and ROPE, the letter E on both would fall over a > symbol on the 340 Cipher.
Taking the second diagram into account it is apparent the letter E in the word SLAVES would fall over a < symbol.
The E in the vertical word PARADICE would fall over a < symbol as well.
Therefore out of the five E's on the Halloween Card, four would fall over a directional symbol, either < or >.
If we transposed this E onto the fourth symbol of the entire 340 Cipher, the word HERE is formed.
This would now connect five directional symbols to the letter E.
It was always strange how his cipher began with an actual word, possibly this was intentional and not there by chance.
In the above example three E's are represented by > symbols and two E's are represented by < symbols. But it would be nice to have five the same.
Well, it may be possible.
The 'Halloween' card has many interesting features, but in terms of focusing in on individual alphabetical letters he does two things. He reverses the letter N in By Knife and underlines LAV in Paul Averly on the envelope.
If we rotate the reversed letter N, 180 degrees to form a correct N and therefore rotate all the symbols on the 17th line of the 340 Cipher along with it, the < (E) of PARADICE becomes >.
If we rotate the LAV, that would be present, in SLAVES shown on the 10th line of the 340 Cipher, the < (E) of SLAVES becomes >.
Therefore all five E's are now represented by the symbol >.
He later sent us the 'SLA' letter mailed to the San Francisco Chronicle on February 14th 1974. Not only did the date of the 'SLA' letter, FOURTEENTH match the 4-TEEN of the Halloween Card, but the date of VALENTINES DAY was also relevant.
If VAL is reversed we get the LAV of PAUL AVERLY, present on the envelope..
The letter N is the FOURTEENTH or 14th letter of the alphabet and this letter is reversed on the 'Halloween' card also.
This is the reasoning behind rotating the symbols on the 10th line of the 340 Cipher.
The SLA reference is another link to SLAVES and the 'Halloween' card, along with the 10th line of the 340 Cipher.
The 'Halloween' card has the word BOO placed on the inside of the card. This is situated on the 14th line of the 340 Cipher. The number 14 is relevant to both the 'Halloween' card and 'SLA' letter and the word BOO is also reversed on the 340 Cipher.
There are possibly additional links from the 340 Cipher to the 'Exorcist' letter, shown by the blue squared section on the first diagram. See here.
Plus a further link to the '13 Symbol' cipher, shown at the bottom of the first diagram. See here.