In solving the 340 cipher, I think the guy in Australia had over 650,000 different possibilities. What are some of the more common methodologies as far as read direction? You could do spiral directions, you could take rows of text and swap them with other rows, etc.
I don’t know much about cryptology in general but I think the killer left lots of clues; some of which might reveal additional reading or transposition directions…
If we interpret the postcard as a hint about z340:
– 13 is either the transposition period or the column that forms the vertical mirroring axis in z340 (reverse and invert the symbols on one or either side)
– the cross under the 13 is the central "+" in z340 (maybe the grid dimensions also need changing before the correct "+" lands in the middle)
– the "Zodiac" is the "ZODIAKI" in the lower right quadrant of z340
– the upside-down text in the lower-left quadrant means that we should read or transpose that part of z340 in reverse
