Due to Oranchak’s great webtoy, there is a really cool tool to analyze Z’s 340/408 ciphers. Credit..

Besides an analysis of ciphertext 3-grams, however, the analysis of cleartext 3-grams might be of interest (408). For this reason I used the corrected cleartext (without fillers) and split them up:

AUS3,ANK,AME2,ANI2,ANG2,ALO,ALL2,AGI,ART,ATW,ARA,ADI,AND,AVE3,AFT
BEC4,BET,BES,BER,BOR
CAU3,CHF,CEI,CKS,COM,COL,CTI
DGA,DAN,DIE,DIS,DAL,DWI,DOW
EKI2,EOP,EBE3,ECA3,EIT2,EFU,EIN,EFO,EST2,EMA,EMO2,ERO,ETH2,ESM,EXP,ERI,ENC,EVE,ENB,ETT2,ERT,ENI,EIW,ERE,EBO,EAN,EIH,EDW,ECO,EMY, ESI,EYO2,ECT,ESF,ERL
FUN2,FOR2,FAL,FFW,FWI,FIT,FSL,FTE
GPE,GWI,GAM,GER,GGI,GIV2,GEX,GET,GYO,GIR,GOF
HFU,HAN2,HEF,HEM2,HIN,HRI,HAG,HEB,HAT,HEN,HEI,HAV
ILI,IKE,ILL10,ING6,ITI4,ISS,ISM,ILD,INT,IST2,IMA,IVE2,IEN,ISE2,ITH,IRL,IDI,IEI,IWI2,INP,IHA,IFE
KEK,KIL4,KSO
LIK,LLI3,LIN3,LEB,LDG,LOF,LLT3,LTO,LLS,LSO,LTH2,LLB2,LBE2,LLE2,LED,LAV2,LLN,LNO,LTR,LOW,LEC,LIF
MUC,MOR,MEI,MAN,MOS2,MAL,MET2,MEM,MYS,MYN,MEB,MYC,MYA
NGP,NIT,NTH2,NKI,NGW,NIS,NGE3,NIM,NGG,NCE,NBE,NGY,NID,NIN,NPA,NDA,NOT,NAM,NOR,NGO
OPL,OMU,ORE,ORR,OST2,OUS,OFA,OKI,OME2,OUR,OCK,OFF,OFI,ORN,OTG,OUM,OUW,OSL,OWD,OWN,ORS,OPM,OLL,OFS,ORM
PEO,PLE,PER,PAR2,PMY
REF,RRE,RES,ROU,RIL,RIE,RTH,RRO,ROC,RLT,RTO,REB,RNI,RAD,RYT,RST,RMY,RLI
SEI,SSO,SOM2,SMO,STB,SEM,STH2,STD,SAN,SME,STT,SEV,SOF,STP,SEA,SLA2,SIW,SEY,SLO,STO,SFO
TIS4,THA4,THE5,TBE,TDA,TOK,THI,TTH,THR,TTE,TER2,TTI,TIN2,TPA,TOF,TWH,TGI,TRY,TOS,TOP
USE3,UCH,UNI,UNT,USA,URR,UMY,UWI
VES3,VEN,VEK,VEY
WIL5,WIT,WHE,WDO,WNO
XPE
YOU3,YSL,YNA,YTO,YCO,YAF

As you can see, most of the text consists of unique trigrams. As soon as a word repeats, e.g. ‘BECAUSE’, its trigrams do repeat as well. If we delete repeating words such as
Because fun kill most than will all itis give you slaves for
we do get a ‘true’ Zodiac trigram structure of his cleartext:

AME2,ANI2,ANG2
EBE2,EST2,ETH2,ETT2
ILL3,ING5,ISE2
LLI3,LIN3,LLT2,LTH2,LBE2,LLE2
MET2
NGE3
OME2
PAR
SOM2,STH2
TIS4,THA3,TIN2
VES2
WIL2

Apart from that, the following are the most frequent trigrams:
ILL,ING,THE,THA,TIS,YOU,VES,BEC,CAU,AUS,USE,IVE,KIL,ITI,NGE,LLI,LLT,LIN,EBE

Repeating words influence the trigram frequency distribution..besides that LL, THE, THA and ING as well as ITIS structures appear to influence such distribution as well. YOU does so,too, obviously as it is a letter addressed to its reader. Leading ‘common word’ trigrams are ILL, ING and THE, followed by WIL, THA, ITI and TIS.

QT