Reconstructing the Zodiac Z13 Through 408 Alphabet Overlap

This preprint documents a reconstruction workflow for the Zodiac Killer’s Z13 cipher based on structural overlap with the 408 solution set. Rather than approaching Z13 as a free standing cryptogram, this method treats it as a constrained micro variant within a consistent internal alphabet already expressed in the 408. The approach produces a closed candidate message space and resolves the long standing interpretive ambiguity around Z13 through phonotactic limits and symbol compatibility. This paper outlines the mapping procedure, constraint testing, and justification for chosen symbol values, and positions Z13 as a deliberately compressed expression nested inside the larger Zodiac cipher architecture.