I too have something to contribute to this major breakthrough.
Remember this book: Codes and Ciphers By John Laffin?

Published in 1964 and featured the Zodiac Alphabet. (also available in the Riverside Library)
doranchak actually has a thread about it here:
http://www.zodiackillersite.com/viewtop ... =81&t=2915
Long story short. The 340 solution appears to match up to the Skytail method of encoding featured in the book.
https://www.dcode.fr/scytale-cipher
The first part (9 lines / 153 characters) of the message encodes to (***9 Turns***):
IRONCAOOIIERGRTMIHETTATNWNNIAABWECOHSRTWTWGTAISEITEPAOYCAHHSAMDCCLOAEVFIHSEIUBNOHALPLYIFNMNTCPOOFAUSALONUGETVHAUTTMSERTUGNTTMSBPTAHBENA·ALIOHEHROMFEEIDD·
I have not fully verified this, but it seems to match the key provided by doranchak.
https://i.imgur.com/Ys2mpXe.png
