The code breakers said that discovering Z340 was three sections was "luck" but it's likely they were informed by previous zodiac antics: "On July 31, 1969, Zod

The code breakers said that discovering Z340 was three sections was "luck" but it's likely they were informed by previous zodiac antics: "On July 31, 1969, Zodiac sent his first cipher: the famous three-part cipher Z408. The Z408 was separated into three different parts..."