Idea/scope of thread: the Z340 may not be solvable as a whole but it may contain a part or parts (sub strings) that are solvable. For example, Olson considered lines 1, 2, 3, 11, 12 and 13 to contain a solution plain text somehow.

To start work in this direction I extracted all continuous sub strings of the Z340 with a multiplicity 0.3 to 0.35. I first did the same with the Z408 to have a control. With AZdecrypt’s Batch substitution it is possible to measure the accuracy of the solution given that the plain text is known:

cipher_information=1-149_348
9%P/Z/UB%kOR=pX=B
WV+eGYF69HP@K!qYe
MJY^UIk7qTtNQYD5)
S(/9#BPORAU%fRlqE
k^LMZJdrpFHVWe8Y
@+qGD9KI)6qX85zS(
RNtIYElO8qGBTQS#B
Ld/P#B@XqEHMU^RRk
cZKqpI)Wq!85L
solution_plaintext=
ILIKEKILLINGPEOPL
EBECAUSEITISSOMUC
HFUNITIAMOREFUNTH
ANKILLINGWILDGAME
INTHEFORRESTBECAU
SEMANISTHEMOATDAN
GERTUEANAMALOFALL
TOKILLSOMETHINGGI
VESMETHEMOATT

Excluding the last 18 characters of the Z408 there are 6104 sub strings with a multiplicity of 0.3 to 0.35 (very redundant). I now needed to know how many iterations and restarts to consider for the test to get a really high average solve accuracy for the Z408 sub strings. So I ran multiple tests at increasing number of iterations:

500,000 iterations: 21.37% of ciphers solved with accuracy >= 70%
1,000,000 iterations: 33.60% …
2,000,000 iterations: 51.09% …
4,000,000 iterations: 67.59% …
8,000,000 iterations: 81.70% …
16,000,000 iterations: 91.30% …
32,000,000 iterations: 97.00% …
64,000,000 iterations: 99.18% …
128,000,000 iterations: 99.75% …
128,000,000 iterations with 10 restarts: 100% !!!

Takeaway is that AZdecrypt was able to achieve a 100% solve rate on the Z408 sub string ciphers using 6-grams but it took almost 2 days.

And now I could proceed to the Z340 sub strings with nearly 100% confidence in the results. There are 5621 Z340 subs strings with multiplicity 0.3 to 0.35.

And for the results. The highest AZdecrypt score of the Z340 sub strings is 22909.94 while the lowest score of the Z408 is 23528.00. So that does not look good to start with.

The best result:

Score: 22909.94 IOC: 0.0640 Multiplicity: 0.3496 Hours: 14.59
Repeats: THEREI STHE OTHE OTH EAR DIS PLE WAS EAN IGH AST AN
PT-to-CT cycles: 117

63-225_349

LIINDOTHEISPLEAST
BIGHUGEANDTALINEB
UTFORYESIDISCOVER
CONSTAYCOMPLELITW
ASINTHEREISIMNOTH
EROTHDISAVOIDMILE
ARTHRIVINGSPEARAH
ESWEDLOREANMANSWH
ENIGHTSTHEREIWAST
HEYCANACTW

And this result almost has a full BECAUSE repeat:

Score: 22488.64 IOC: 0.0631 Multiplicity: 0.3442 Minutes: 26.47
Repeats: BECAUS THIS (2) ETTH STOR RES EVE ORE ORM ER IN OM
PT-to-CT cycles: 225

3-185_344

APTORMBUILDYOULIT
ESTORINAGAINTHIST
HEVERCOATIDONTGET
THROWNMYTREEATUSE
VENAFLIMSBECAUSTW
ARSORONLYUSPUNTOM
ETHISURESOMANDITO
UGHFORESIGNEDDELI
VEBECAUSSETTHATON
ORMERGERTHISSTORE
ASMAKINGSTBYV

Conclusion: the Z340 does not contain any solvable multiplicity 0.3 to 0.35 continuous sub string on par with the Z408.

Further work would be to try multiplicity ranges 0.35 to 0.4 and 0.4 to 0.45 etc.