As can be seen on the moon calendar, the full moon phase on this occasion lasts four days from April 19th to April 22nd, paralleling the four unusual, but as yet undetermined symbols on the cipher. As noted above, they could possibly be three Taurus signs enclosed in a circle (the moon), - and one Aries sign. In astrology the Zodiac sign of Aries on average runs from March 21st to April 19th, with the Taurus sign taking 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 effectively we have 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.
The last two potential Zodiac attacks prior to this mailing were the Kathleen Johns abduction on March 22nd 1970 and the double murder of John Franklin Hood and Sandra Garcia om February 21st 1970. Both fell on a full moon. Check the calendar for confirmation.