Jelberg:

Indeed, thats weird.

I wanted to buy the book "The Codebreakers", but I haven’t done it yet. I have a lot of books on the subject now, but this one is still missing in my collection.

Some people say it’s useless for Zodiac cipher research due to it having so much historical information, but I disagree.

There are so many instances of things that Zodiac referenced in the book, that I can’t imagine he didn’t read it.

I have a similar view to yours, though not to the same extent.
Perhaps many of these similarities are coincidence. For example, I don’t think the 4-teen on the Halloween card and the Porta disc have a connection. Other things, however, such as the matching symbols are very conspicuous. I definitely felt it is important to read the cipher books of that time. Zodiac will probably have done the same and perhaps this will lead to a new idea.
Looking at the statistics of z340 is still the most important aspect if you want to find a (hopefully existing) solution at some point. Nevertheless, one should also consider the many other aspects, such as the connection to books.

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