"Kibrit: Are you interested in the possible conclusion about the author of the letters? ... All three letters were typed in a quiet home environment. The typewriter is Moscow, old one, has not been cleaned for a long time. It's printed somehow, nonprofesional, by same person. It's a woman, young or middle-aged."
January 1, 1970
https://en.wikiquote.org/wiki/Investigation_Led_by_Experts