Was the Zodiac Killer was just playing games with The American Cryptogram Association, determined not to have a further two ciphers cracked in a matter of days? - and thereby avoiding the acute embarrassment of creating "a cipher designed by man that can be solved by man". After the challenge on October 22nd 1969, the Zodiac Killer was going to design the remaining ciphers on his terms, to which he would ultimately reveal the answers. He wasn't going to allow Dr. Marsh the satisfaction of using cryptographic techniques to pry open his ciphers, so used the wording of Dr. Marsh "to send a cipher code, however complicated" against him, by sending two of the most infantile ciphers possible.
If we strike out the seven letters that have now formed "NAME" we are left with "NAME K". If this was the beginning letter of his surname, then the first circled 8 could represent his forename. So, if we are looking for an expanded declaration of his name using both forename and surname in later communications, we would be looking for a two letter combination, ending in K. But in addition, this would have to be accompanied by some pertinent text alongside the two letter declaration. This occurs in only one subsequent communication of the Zodiac Killer.
If "Fk, I'm crackproof" was the answer to the 13 Symbol cipher, then one would have expected the Zodiac Killer to have designed the cipher with some markers present, that the answer could be fitted around later. Especially if no cryptographic technique was required. The phrase of "Fk, I'm crackproof" was the perfect response to Dr. Marsh, in that Zodiac could satisfy the request of Dr. Marsh by giving us his name, albeit in the shortened format. He began the April 20th 1970 letter by stating "By the way have you cracked the last cipher I sent you? My name is------". He responded immediately with his name (FK), before rubbing Dr. Marsh's "noze in his booboos" by declaring he was crackproof, having also just taunted the cryptographic expert with the mocking introduction of "have you cracked the last cipher I sent you?". The Zodiac Killer knew that Dr. Marsh would never solve his Z340 and Z13 ciphers when he designed them, because they couldn't be solved using standard cryptographic methods - and more importantly - the answers were 6 months in the making.
The circled eights (The number 8 in the Bible represents a new beginning) seemed to have extra significance compared to the other characters in the cipher. Because the Celtic Cross and Anchor have relevance in Christianity, it was considered that the number 8 was the beginning of each word. The three circled eights, along with the K and M inside them, were the markers I described earlier, so that his later solution would be understood. All we had to do now, was place the phrase "Fk I'm crackproof" into the 13 Symbol cipher, with each number 8 beginning each element of the phrase.