It goes without saying that the Zodiac Killer often referenced, or was triggered by recent newspaper articles he had read - so the idea of the "suicide's grave" could have germinated in the mind of the Zodiac Killer just before he mailed The Exorcist letter on January 29th 1974. Therefore, what are the odds of the "suicide's grave" being mentioned in the newspapers the day before "The Exorcist" letter was postmarked, that tied into the Japanese themed "The Mikado" (Tit-Willow) and the Asian style characters at the foot of the letter? In total, we would have "The Exorcist" movie and the "suicide's grave" of demonic possession, the Tit-Willow verse from a Japanese satirical comedy, a Japanese relevant newspaper headline on January 28th 1974 referencing the "suicide's grave" of Kao Kang and some Asian symbolism arranged at the bottom of The Exorcist letter. The content in the article below appeared in several newspapers, but this one was published on January 28th 1974, with the inset image (bottom right) from January 26th 1974. They tell of the demise of Kao Kang to his suicide's grave.
The Japanese connection to the Zodiac Killer was argued in the article The Suffering Mother Reborn Into Paradise, where it was shown that the phrase "reborn in paradise" from the 408 cipher was a product of Japanese culture and history. This, along with the Zodiac Killer's choice of The Mikado in the Little List letter (July 26th 1970) and The Exorcist letter (January 29th 1974), and the findings presented in this article regarding the "suicide's grave" of Kao Kang, the January coverage of China's sought accord with Japan over the threat from Russia, along with the demonic possession featured in the James Hogg novel (Suicide's Grave) and "The Exorcist" movie, creates a contemporary connection with the past.