Don’t agree with everything you stated but will only address the following:

"Last night" would imply the letter was written on Sunday, October 12, 1969. The postmark for Monday, October 13, 1969 in the "p.m." indicates the letter was already in a sectional center between the hours of 12 noon and 5 p.m. Given that, it takes time to bag and deliver mail by truck to a sectional center, the letter would have had to have been mailed in the morning on Monday. How early is really anyones guess right now as we dont know at what specific times a post office stops bagging and begins transporting mail. They do it several times throughout the day based on the following:

Letters received by the sectional center between 12 a.m. and 12 p.m. are postmmarked a.m. and those received between 12 p.m. and 5p.m. are postmarked p.m. A letter arriving at a sectional center between 5 p.m. and 12 a.m. is postmarked -p.m.

Lake Berryessa mail would have gone to the Oakland Sectional center. In essence and terminology it could be seen as "North Bay". Probably doesn’t mean anything. So far I can’t justify it. But I find it interesting that one of the Sectional Centers in California is named "North Bay" (Petaluma).

Soze

"I am the murderer of the taxi driver over by Washington St + Maple St last night, to prove this here is a blood stained piece of his shirt".
If we dissect this statement, bearing in mind the taxicab travelled one block over from the destination purportedly written on the trip sheet, Washington and Cherry is not only by Washington and Maple, but it is over in respect to the movement of the taxicab. I know certain elements of the internet are forever claiming the shirt piece was not proof the letter writer was the killer (not many admittedly), but the October 13th writer named the intersection on the trip sheet. One has to ask, why didn’t the letter writer state "I am the murderer of the taxi driver over by Washington St + Arguello Boulevard last night, to prove this here is a blood stained piece of his shirt".

By October 13th 1969 the direction of travel of the taxicab was unknown to the general public. The October 12th Chronicle article only stated "Company officials said Stine apparently picked up a fare on the street while en route to Ninth Avenue." The letter writer could in effect have written over by Jackson and Cherry, Clay and Cherry, or Washington and Arguello, but picked Washington and Maple – exactly what was written on the trip sheet. It could be argued he chose "over by Washington and Maple" as further proof he knew the original intended destination of the taxicab.

I should add that the San Francisco sectional center is on Evans Street. The farthest south for a San Francisco center portal is roughly Sunnyvale. How far is it driving time from Sunnyvale to Evans? Knowing that time would give you the latest time the Zodiac would have mailed his letter but still isn’t exact. That would just cover driving time and not bagging.

Soze

Edit: quick look and it’s 1hr and 12 minutes drive.

"I am the murderer of the taxi driver over by Washington St + Maple St last night, to prove this here is a blood stained piece of his shirt".
If we dissect this statement, bearing in mind the taxicab travelled one block over from the destination purportedly written on the trip sheet, Washington and Cherry is not only by Washington and Maple, but it is over in respect to the movement of the taxicab. I know certain elements of the internet are forever claiming the shirt piece was not proof the letter writer was the killer (not many admittedly), but the October 13th writer named the intersection on the trip sheet. One has to ask, why didn’t the letter writer state "I am the murderer of the taxi driver over by Washington St + Arguello Boulevard last night, to prove this here is a blood stained piece of his shirt".

By October 13th 1969 the direction of travel of the taxicab was unknown to the general public. The October 12th Chronicle article only stated "Company officials said Stine apparently picked up a fare on the street while en route to Ninth Avenue." The letter writer could in effect have written over by Jackson and Cherry, Clay and Cherry, or Washington and Arguello, but picked Washington and Maple – exactly what was written on the trip sheet. It could be argued he chose "over by Washington and Maple" as further proof he knew the original intended destination of the taxicab.

Yes, this aspect puzzles me. First, why ask for Wash and Maple and then end up a block north? Did Stine take him to the wrong location? Was he ordered to go by Zodiac? Why would Zodiac tell him one thing and then pull a gun and order him to do another? Was Zodiac mistaken originally and wanted to be more north to be closer to the park to hide? If so, why not ask to be dropped off next to the park rather than a few blocks away?

Very curious.

Don’t agree with everything you stated but will only address the following:

"Last night" would imply the letter was written on Sunday, October 12, 1969. The postmark for Monday, October 13, 1969 in the "p.m." indicates the letter was already in a sectional center between the hours of 12 noon and 5 p.m. Given that, it takes time to bag and deliver mail by truck to a sectional center, the letter would have had to have been mailed in the morning on Monday. How early is really anyones guess right now as we dont know at what specific times a post office stops bagging and begins transporting mail. They do it several times throughout the day based on the following:

Letters received by the sectional center between 12 a.m. and 12 p.m. are postmmarked a.m. and those received between 12 p.m. and 5p.m. are postmarked p.m. A letter arriving at a sectional center between 5 p.m. and 12 a.m. is postmarked -p.m.

Lake Berryessa mail would have gone to the Oakland Sectional center. In essence and terminology it could be seen as "North Bay". Probably doesn’t mean anything. So far I can’t justify it. But I find it interesting that one of the Sectional Centers in California is named "North Bay" (Petaluma).

Soze

But wouldn’t this still add weight to an argument that it was not posted until the Monday, seemingly the day after the letter had been written ?

Just an opinion here:

If he mailed it Sunday he likely mailed it in a street box not marked for Sunday collection. If he mailed it at a po on Sunday in one of their drop slots it likely would have had an a.m. postmark. So he didnt do that. Postal employees are at po long before po opens and are processing mail then. I’ve seen trucks in my area picking up before po opens. If he deposited mail in a street box on Monday he did it early. No clue how early he could have done it. In my opinion I think he walked into the po and dropped the letter in the slot at a po rather than handing the letter to a po employee. The delay between killing Stine and mailing letter:

1. He needed sleep.
2. He needed time to think about what to write that doesn’t give him away
3. He didnt know of a Sunday pick up location
4. He didnt know, like a good many Americans, that the po runs 7 days a week
5. He may have figured he would drop it off on his way to work Monday, signifying at least, a monday through Saturday job.

Lots of reasons he could have had not counting family obligations. He could have been like some Americans that drink heavily on a Saturday night and went to church on sunday except, instead of drinking, he killed. Lots of reasons he could have had.

Soze

It is my guess, that the intersection of Washington and Maple may have had pedestrians by the intersection, and knowing he had murder in mind, simply took Stine to the next intersection to facilitate his escape. From here he effectively backtracked one block east, indicating he had originally intended to murder Paul Stine at Washington and Maple and head north up Maple.
A turn east into Jackson (out of sight at the quickest possible juncture) and just one block east to Spruce Street was likely his original intended escape route. It’s where he ultimately ended up being observed by eyewitnesses bordering Spruce, as detailed in the October 12th Chronicle article.

Richard, I’m not sure how much stock we can put into the facts of the Oct 12 newspaper article. My guess is a reporter at his desk at the Chronicle picked up the crime on a scanner and made a phone call to an SFPD spokesperson or source, and possibly one to the cab company. Deadline for the Sunday edition would have been tight – likely midnight Saturday night or earlier.

It is my guess, that the intersection of Washington and Maple may have had pedestrians by the intersection, and knowing he had murder in mind, simply took Stine to the next intersection to facilitate his escape. From here he effectively backtracked one block east, indicating he had originally intended to murder Paul Stine at Washington and Maple and head north up Maple.
A turn east into Jackson (out of sight at the quickest possible juncture) and just one block east to Spruce Street was likely his original intended escape route. It’s where he ultimately ended up being observed by eyewitnesses bordering Spruce, as detailed in the October 12th Chronicle article.

Seems to make sense Richard, especially if you consider that directly opposite the end of Cherry there is a footpath which leads into the Presidio. If he had want to go that way surely that would have the quickest way and not to double back towards the Julius Khan playground. Also beginning at Spruce on W Pacific you have the parking area. I do wonder that if he was not overly familiar with the area, could the Julius Khan playground be the reason why he had slight knowledge of it.

Richard, I’m not sure how much stock we can put into the facts of the Oct 12 newspaper article. My guess is a reporter at his desk at the Chronicle picked up the crime on a scanner and made a phone call to an SFPD spokesperson or source, and possibly one to the cab company. Deadline for the Sunday edition would have been tight – likely midnight Saturday night or earlier.

The problem I see Xcaliber, is "guessing a reporter at his desk at the Chronicle picked up the crime on a scanner" is less reliable than the essence of the details presented in the newspaper article – and as you say, is just a guess. While newspaper articles aren’t always wholly reliable sources of information – the notion of the killer being spotted entering Julius Khan playground does have some credence when we consider the directional movements observed by one of the teenagers (Lindsay) and later by Officer Donald Fouke on Jackson Street. Spruce Street being the first thoroughfare into the park. While the Zodiac could have clambered around bushes and through Cherry or Maple, these were not visible at 10:00 pm at night, and only somebody heavily familiar with this particular stretch of Jackson and West Pacific Avenue would be aware of them. This, however, doesn’t mean Zodiac was unfamiliar with the area to some degree. If you don’t put much stock into this newspaper article, then there has to be a reason why you have arrived at such a conclusion.

Richard, I don’t doubt that the killer ran into the Presidio, but my point is that the Oct 12th newspaper article would not have been thoroughly researched, and the reporter would not have been on the scene. The ‘Later reports indicated someone was seen running into Julius Khan playground’ is vague, and doesn’t point to witnesses.

Separately — the police would have been able to determine (and announce) the apparent murder weapon that quickly?

Thanks X, and I hadn’t forgotten your contribution to the park analysis. I don’t know what validity you ascribe to Graysmith, but he stated "The detectives heard from neighbors that a stocky figure was seen dashing across Julius Khan playground and into the dense undergrowth of the Presidio. The dog patrol units, seven of the best search dogs in the country, gathered at the front entrance of the Presidio and were deployed one at a time in various directions. Armstrong and Toschi considered the possibilities. Had the killer gone quickly through the dark woods and emerged from the Presidio at Richardson Avenue, and taken Highway 101 past Fort Point onto the Golden Gate Bridge and vanished into Marin County."

Richard, that sounds logical, that if the detectives canvassed the neighborhood they would have run across residents who saw the man fleeing — or the residents might have simply contacted police.

I think we both agree that the Richardson Avenue escape was impractical and unlikely, and I wonder if the detectives were aware of, or considered, the Broadway gate.

But again — re the Chronicle article — I can’t see the time frame working, where all that information — such as witnesses seeing the man fleeing across the playground — would have been gathered in time to make that night’s Chronicle deadline.

(Graysmith also may be over-stating the prowess of the dogs. I believe they were German Shephards that rode in the back of select squad cars, typical of city police departments, but not specialized tracking dogs.)

This clearly shows that there was no sexual motive behind the attacks, the attacks served his need for power. He had power over peoples life, again this can be seen by the escalation for the attacks.
1st Attack – Shot and Ran
2nd Attack – More Interaction with the Victims, Phone call and letters
3rd Attack – Lot of interaction with the Victims, First day light attack, Costume, Writing on car door, Phone call
4th Attack – Full control over the attack, even to the point of controlling the victims last few minutes on the planet (by this I mean he was a taxi driver and he is doing your will as he is driving you to where you want to go), attack in a major city, letter, taking evidence of the crime.
Letters up to this point had been matter of fact, I did this, with this, and this happened. He obviously got something from the Cipher, but he must have been upset when it was cracked for easily and quickly.
Now that he could fulfil his need in a different way we see that his next two letters were significantly different from anything previously.
Instead of a standard letter we now get a greetings card and a 6 page rant with diagrams of his bomb.

One can argue that he had infinity times fold more communication with Betty Lou Jensen and David Faraday upon getting them out of the car in Lake Herman Road in order to kil them, in comparison to his "shoot while mute before u run" attack on Mike Mageau and Darlene Ferrin in Blue Rock Springs some 6 (almost 7) months later.

Back in Lake Herman Road he must have showed up out of nowhere while the two young lovers were too occupied to give his presence some attention, then hopped out of his car with his flashlight gun, shouted towards them to get out and shot one bullet in the air, maybe pretending to be a robber like in Lake Berryessa 9 months later, then he proceeded to shoot above the rear window since they were frozen still, and once they weren’t getting out after that try he shot the window, then Betty Lou Jensen got out first and once David Faraday was about to get out of the vehicle or that he was already out but haven’t fully stood up, the zodiac shot him from close range, seeing this Betty Lou was terrified and began to run for her life, that’s when he shot her in the back 5 times (the sixth shot missed IIRC).