According to Dean:
“In the earliest couple of years of his career, Dean wrote a few letters and articles for science-fiction fanzines. He was not prolific in this area because he was too busy writing fiction to pay the bills and to learn his craft. Therefore, in 1991, Dean was shocked to learn that a person he had previously worked with professionally had, beginning in 1969 and continuing at least through the early 1970s, been writing letters in Dean’s name to individuals and had submitted letters, and even some articles, in Dean’s name to fanzines. The name “X” will do until the full story can be told in Dean’s memoirs. All of this information was first disclosed to Dean in 1991 when X provided a written admission of these activities, although he could not remember everyone to whom these forged letters and articles had been sent. Consequently, any fanzine appearances by Dean after 1968 are highly suspect unless they were submitted with a cover letter on his own letterhead of that time.”
Source: https://www.deankoontz.com/about-dean/collectors