This letter contained the 13 Symbol Cipher, mailed to the San Francisco Chronicle on April 20th 1970, just one month after the Kathleen Johns incident on March 22nd 1970.
Therefore in the calendar above there is one full moon phase in Aries, before the Zodiac sign of Taurus takes over on April 20th, 21st and 22nd. This may explain the 3 to 1 configuration present on the 13 Symbol Cipher. We now have seven dates of relevance, March 22nd 1970, April 19th, 20th, 21st and 22nd, July 24th 1970 and March 22nd 1971, but whether any significance can be attributed, remains questionable.
The March 22nd 1971 Pines Card may also have strong ties to the Zodiac horoscope signs, in its very wording and ultimately provide us with a further link back to the 13 Symbol Cipher and its three circled symbols once again. SEE HERE.
The following observation will not claim a solution, just a line of thought to be considered for discussion.
In this letter the Zodiac Killer claimed to have 'killed ten people to date'. Had he murdered Kathleen Johns and her infant daughter Jennifer, this total would have read 12.
Kathleen Johns was abducted on March 22nd 1970 and on the following anniversary of this crime, March 22nd 1971, the Pines Card was received at the San Francisco Chronicle, claiming to have 'Sought victim 12'.
There are four crucial dates at play here. March 22nd, detailed twice above, April 20th 1970, the mailing date of the 13 Symbol Cipher and July 24th 1970, the date of the correspondence the Zodiac Killer mailed, claiming responsibility for the abduction of Kathleen Johns. Bearing in mind the Zodiac horoscope signs, possibly present on the 13 Symbol Cipher, were the dates on the Zodiac calendar in any way relevant to these events.
Firstly April 20th, the 13 Symbol Cipher date is the beginning of Taurus on the Zodiac calendar. The abduction of Kathleen Johns on March 22nd 1970 is one day after the beginning of Aries on the Zodiac calendar. The Kathleen Johns letter on July 24th 1970 is one day after the beginning of Leo on the Zodiac calendar. Three horoscope dates, all within one day of the Zodiac Killer timeline. Four, if you include the Pines Card anniversary.
The 13 Symbol Cipher was sent in the April of 1970 to the San Francisco Chronicle, so let us look at at the Moon phases and lunar calendar of San Francisco from the relevant time period. But before we do, it is important to note that the Zodiac Killer circled his three Taurus symbols and the symbol for a full moon, is basically a white circle.