It is a possibility the thirteen symbols present within this cipher are there because of a fascination towards the 'unlucky 13' number and reasons detailed here: http://www.zodiacciphers.com/1/post/2013/01/the-zodiacs-little-list-of-13.html.
As has been noted before the three circled 8's could in fact be Taurus symbols and the 10th symbol an inverted Aries sign, the Zodiac sign is nothing unusual for the killer and as for the eight alphabetic letters, well that's another question..
Here are the moon phases of San Francisco for April 1970, after all this letter was mailed on April 20th 1970. SEE CALENDAR HERE .
It is important to highlight that a full moon is really only full for an instant in time, as we approach a full moon it is said to be waxing and as it diminishes it is said to be waning. A Waxing Gibbous denotes 95% illumination approaching a full moon and a Waning Gibbous denotes 95% illumination as we leave the full moon, everything in between can be regarded as varying degrees of a full moon.
As can be seen on the moon calendar, the full moon phase on this occasion lasts four days from April 19th to April 22nd and there are four unusual as yet undetermined symbols on the cipher.
As noted above they could possibly be three Taurus signs enclosed in a circle, which could represent the moon and one Aries sign.
In astrology the Zodiac sign of Aries runs on average from March 21st to April 19th and Taurus takes over on April 20th ( the date this letter was mailed ) until May 20th. Although these dates can marginally change over the years and determining your sign near a cusp, can require a little extra help.
So there is one full moon phase in Aries on April 19th and three full moon phases in Taurus on April 20th, 21st and 22nd, hence the Zodiac's use of 4 symbols in a 1:3 configuration.
Possibly the cipher was meant to run backwards, not unlike other cards, as the Taurus signs are to the left of the Aries sign,and the upside down Aries sign is supposed to point us in that direction, after all if it were reversed it would look the same.
Having said that it is conceivable that the Taurus signs are also upside down.
This leaves us having to determine from right to left ( or the reverse), the reasoning behind the alphabetic symbols MAN MK [ zodiac sign ] NEA.
Obviously by rearrangling the letters you can get KEN, then its a matter of juggling the remainder to arrive at KEN AMMAN, the Zodiac killers name, as shown here by Leroy Blevins on Youtube.
He states the non alphabetic symbols have no meaning, but that is where we differ.
The Zodiac had an affinity towards astrology and the moon in particular, so they meant something to him, may'be a birth date or a relevant date in his timeline of murder, but without finding a year makes this difficult, although April could still be significant.
Ken Amman can be found on Google, but is KEN AMMAN the Zodiac.
The 340 Cipher is complex but finding Ken Annam or indeed Kenneth Annam from the 'My Name is Cipher' was not hard and therefore one cannot believe he would undo all of his 'hard work' up to this point with a cipher that hardly lives up to its name.