Some people use the 'evidence of absence' argument to suggest the Lake Berryessa attacker was not the Zodiac Killer- articulating that his lack of mailed correspondence after the double stabbing was evidence for his absence from the crime scene.
His communication on the car door just six minutes after the attack- probably to him- seemed like unassailable proof he was placing himself at the crime scene, just like attaching Paul Stine's shirt piece to his October 13th 1969 letter.
At Lake Berryessa he took his handwriting to the crime scene, while at Presidio Heights he took the crime scene to his handwriting. A reverse signature one might say. However, not many people question Zodiac's involvement in the murder of taxicab driver Paul Stine. The idea that the Zodiac Killer never communicated his involvement in the Lake Berryessa crime is patently false, despite our need for a more comprehensive attribution to the events of September 27th 1969. Yet, his correspondences on October 13th 1969 and November 8th 1969 may very well have both referenced the Lake Berryessa attack, but in different ways.
The author of the October 13th 1969 Paul Stine letter opened with the words "This is the Zodiac speaking. 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. I am the same man who did in the people in the north bay area."
The Zodiac Killer on the car door of Bryan Hartnell's white Karmann Ghia clearly attributes both of his first two attacks under the banner of Vallejo, despite the fact David Faraday and Betty Lou Jensen were murdered in Benicia. In fact, the Zodiac Killer never referenced Benicia in any of his communications, while he mentioned Vallejo several times. If the Zodiac Killer played no part in the Lake Berryessa attack, then his above introduction could have read "This is the Zodiac speaking. 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. I am the same man who did in the people in the Vallejo area." However, he chose to use the "north bay area" for the first time, which incorporates the Napa and Sanoma wine regions.
The divergence from Vallejo in the October 13th 1969 Stine letter confession, therefore, proof of his admission to the Lake Berryessa attack. The Zodiac Killer would then further indicate his involvement by the lake, in the cryptic November 8th 1969 'Dripping Pen' card communication, not only by the addition of "Sept" in his running victim count, but through the cover image and wording within it.
The style of the card may have been purposely chosen in reference to the Lake Berryessa attack and mailed in unison with the 'Bus Bomb' letter, which elaborated upon his recent murder.
On October 18th 1969, Attorney General Thomas C Lynch implored Zodiac to turn himself in, stating "We will see that he gets help and all his rights are protected. He is obviously an intelligent individual. He knows that eventually he will be taken into custody, so it would be best that he give himself up, before tragedy is written in blood."
Three weeks later the Zodiac Killer mailed a greeting card, depicting a fountain pen having been washed by its owner. The card itself read "Sorry I haven't written, but I just washed my pen...and I can't do a thing with it." Zodiac then added the words "This is the Zodiac speaking. I though you would nead a good laugh before you hear the bad news. You won't get the news for a while yet. Could you print this new cipher in your frunt page? I get aufully lonely when I am ignored, so lonely I could do my Thing."
The phrase "and I can't do a thing with it" is referring to a pen that now cannot be used, having been damaged by water. This is often taken as a pen accidentally left in clothing and washed, however, it may have a more sinister implication.
The threat of "I could do my Thing," is obviously referencing murder- but by parroting the word "thing" from the card, which is explicitly referencing the pen, the Zodiac Killer may be associating the pen with murder. More specifically, the murder of Cecelia Shepard.
The Zodiac Killer's dominant hand would certainly have been covered in blood after the Lake Berryessa double stabbing, so any pen subsequently used to write on the car door would equally have been smothered in blood, thereby making the November 8th 1969 'Dripping Pen' card, of a washed pen, not an unusual choice by the killer. The killer may have worn gloves and removed them before writing on the car door, but the implication of the card remains.
The six exclamation marks after the word "Thing" are as notable, as they are excessively repetitive. The Zodiac Killer has also deliberately or accidentally underlined the word "Thing" exactly six times. These two characteristics may be purely unintentional on behalf of the author, but considering the Zodiac Killer is claiming 7 victims at this juncture, it would be remiss not to mention that the Lake Berryessa murder of Cecelia Shepard would have been the Zodiac Killer's sixth victim.
One theory behind the origin of the exclamation mark, is that it is derived from a Latin exclamation of joy, which may not have been lost on a killer bereft of empathy.