I think many of us agree some of the detective magazines of the Zodiac era were most likely an influence to him. The following was shared by a member at Zodiackillersite. I think this is a nice find. While I don't think Harden was Zodiac, if that guy who does the videos found it...kudos to him.
The guy who believes the Hardens were responsobile for the Zodiac murders referenced this in some of his last videos.
In the "Detective Story Magazine" number of August 27th of 1921, the main story on it is titled "Z", about a guy who murders bank presidents and employees. "Z" phonecalls a local newspaper, and presents himself according to the person who picked the call by saying "This is Z speaking".
(The photo is from page 39)