The ‘Forgotten Zodiac Killer Cipher’ Z 148 comprises 8 rows of different lengths of 11 to 25 characters. In total, the cipher contains 148 characters represented by 23 different cipher symbols covering 21 letters of the English alphabet and 1 additional cipher symbol standing for 1 numerical value. The letters "B", "J", "Q", "V" and "X" are not present in the text of the cipher solution and therefore not represented by additional cipher symbols.
Similar to the Zodiac Killer’s 408-character cipher Z 408, the Z 148 is a simple substitution cipher that has been twisted, although in this case this has been done through different means.
In case of the Z 148, the author of the cipher used specific words and repeat words to disguise letter frequencies of the English language. For instance, the letter "I" appears 19 times in the cipher solution, equivalent to 12.84 percent. In Standard English text the letter "I" is only 7.26 %. Another example is the letter "K" that appears in the cipher solution 7 times, equivalent 4.73 %. In Standard English text the letter "K" is only 0.67 %.
This disguise also worked the other way around, for instance the letter "E" that has been underrepresented with only 10 occurrences, equivalent to 6.76 %, while in Standard English text it is 12.51 %. Same goes for the letter "R" with only 2 occurrences in the cipher text, equivalent to 1.35 %, while in Standard English text it is 6.12 %.
What additionally made the cipher difficult to crack were poor quality of the cipher copy, unclear handwriting and therefore unclear cipher symbols, spelling errors in the cipher text, the inclusion of non-cipher text in the actual cipher in case of the letter "G" at C24 and "T" at E5 that, in a way, made the cipher homophonic, as well as the inclusion of the numerical value 3 at F1 that we had not anticipated.
For the above reasons, an actually simple cipher was not exactly easy to decode. While the cipher solution looks straightforward when you understand its key, architecture and twists, it is a completely different story when you sit in front of a bunch of strange symbols without all this information. In some cases, one could not even say if similar looking cipher symbols of the cryptogram are identical or if they are different cipher symbols.
All this may explain why the cipher could not be cracked earlier. Another reason was certainly its comparably low popularity.