Hi, Long time lurker on here!
I’ve been trying to solve the Z340 for around 6 years. Trying the old school pen and paper way, as I think that’s how it was created.
I was on the wrong track, because I thought it was quadrant encoded, and something to do with ebcdic. (Ascii of the late 1960’s) Ah well, we now know!
Obviously beaten to it by people who know waaay more than me about ciphers!
Huge congratulations to Dave, Sam and Jarl.
All the best everyone,
Paul

Now, to solve the last 18 characters of the z408…. hehehe!