Hello! Im from Germany, so I apologize for my bad english.
Im a writer and lector in Germany, so language itself is my focus.

So to the case:

In his first letter to the 3 Papers Zodiac enumerates a couple of things, to prove that he is responsible for the attacks on Faraday/Lou Jensen and Ferrin/Mageau.

He writes:

Christmass
…
3 the boy was on his back with his feet to the car
4 the girl was on her right side feet to the west
4th July
1 girl was wearing paterned slacks
2 the boy was also shot in the knee

It is noticeable, that the article “the” is missing for the discription of Darlene Ferrin. It only shows “girl”, where the three other discriptions begins with the article “the”. From language perspective this indicates that the writer – while writing – thought the sentence “Darlene was wearing paterned slacks” and substituted the name with a neutral noun, while transforming thoughts in writing. This is a common pattern that occur in the process of writing which lectors have to deal with. You can say, that stuff like that shows the subconscious truth of a writers thoughts while writing, which can be a unwanted result, while writing a novel, for example.

This indicates, that the writer knew the name of Darlene, where he did not for the other three people.

Another small indication: For the Christmass-Incident the writer mentions the boy first. For the 4th-July-Incident he mentions the girl first. Just like you would do, if you would know that person. I know, its just 50/50-chance who to mention first. But it combines with the missing article.

Thats all I wanted to mention. Thank you! 

I am very sure Dee knew her killer, she knew he was watching her, and following her, she feared him. I believe he was the man she called Honcho. who went with her and her first husband Jim Philips aka Crabtree. She told her babysitter; she saw him kill someone!