How were the individual characters encoded?
It was difficult to solve because the cypher has three different sections, and they're intended to be read diagonally, but each section also has some additional minor ad hoc rules.
In the video, he notes that 2 of the three sections have the same number of lines (10 iirc) and the 3rd section has only 2 lines.
I wonder if it was just trial and error that made them realize there were 3 sections or if there were "compositional" differences that made each section stand out from the others.
"On July 31, 1969, Zodiac sent his first cipher: the famous three-part cipher Z408. The Z408 was separated into three different parts..."