I believe there is a theme running through most of Z’s letters, and some of the unconfirmed letters. Three 11s equal 33. I believe this may lead to his identity or at least to someone Z is pointing his finger at. In 3 of Z’s letters: July 31, 69 Vallejo Times Herald letter, Button and Kath John’s letters, he mentions the word COUPLE. I believe the word COUPLE, in his mind, refers to all the double letters in his words. Examples: mm in ammo, pp in supply, ll in killing. His letters are full of these COUPLES, especially the double ll. The double ll looks like the number 11. This is the main COUPLE that fits in my observations, along with the letter H,M and N, as the vertical lines on these letters look like the number 11, and sometime the quotation marks and ditto marks. Main idea, things that look like 11 or represent 11.
Belli letter has three words with ll,will, will, all. Same for My Name is letter, killing, killing, killed. Dragon Card, tell, all, well. Citizen letter, killing, Holly, killing.
Sla letter, spell and quotation marks around "sla" The My Name is, second page, three ditto marks on the same line, the middle ditto mark is under the letter K in broken. K is the 11th letter in the alphabet.
Bus Bomb letter, in the PS section, "page 3 or I shall do my thing". Page 5 of same letter, the cardboard tubes on this diagram are a huge 11, and two 6 V bat. V is roman number for 5, so 5+6=11. Three 11s again equals 33.
Dripping Pen, six exclamation marks are three 11s. 3×11=33. Divide 33 into the dots, 231=7. Same number on the bottom right of card.
Red Phantom, Marco, Meanwhile, Marco. Note they look similar to the number 7. The Belle envelope, Mr. Melvin M. Belli.
Halloween Card, Happy Halloween. The H,H and ll are in a triangle shape. Gives balance to the two triangle formed by the skeletons legs. There are 6 lashed on the huge eye. The first two are connected, the next three are the same size, and the last one is half the height of the rest. 231.
Exorcist Letter, Three tit willo’s. His drawing on the bottom. A backwards K, made with 3 strokes=33. The I is one stroke, the F is three strokes, the V is two strokes. When flipped is 231, then divided by 33 is 7. Two symbols on the left, think of the bottom mark representing the number 6 on a clock, the mark above would be the hand, pointing to the number 7.
Little List letter, last page, Mt. and the #. The Mt. is on the circle. Page 2 of the letter, the = marks (side ways 11s) are near or touching the Z symbol.
Button letter, .38. 3+8=11. The cipher has three COUPLES, CC, OO, and two triangles. Add there positions according to rows 1+2+6+9+14+15=47. 4+7=11. No third 11 as it balances with the next page the map. "Is to be set to Mag. N." M and N are 11s.
There are more examples, but I think you get the idea.
Unconfirmed letters, He will be a cop, the K in Ok is placed under the ll in will. There are 38 letters in the cipher below.
Government letter, add all the kills predicted, 3+1+6+8+3+9+8=38. Also the 3 ditto marks under the word ‘times’ =33. The symbols in the drawing, a square has 4 sides, triangle 3, circle cut in half is two parts, and the 5 zodiac symbols are divided into 4 parts each. Add them up, 4+3+2+2+2+4+4+4+4+4=33
Hello it’s me, The two H’s on first line and the ll in Hello, I believe to be the 3×11=33. The one that caught me eye was on the 7th line of this letter, above the letter C in the word certain, there is an 11.
Eureka card, Magnet and 2 keys.
Sorry this post is so long. I tried to explain it in a nutshell.