The Zodiac Killer is allowed to produce handwriting in subsequent letters the same or similar to previous communications because he is effectively doing no more than mimicking himself. A copycat or hoaxer can mimic the Zodiac Killer using previously published letters and envelopes in the newspapers, but what they cannot do is mimic unreleased communications by the Zodiac Killer. To my knowledge, only two envelopes were released to the newspapers prior to January 29th 1974 that were addressed to San Francisco. The envelope to the Melvin Belli letter was released on December 28th 1969 with the address of "Mr. Melvin M. Belli 1228 Mtgy San Fran Calif". The envelope to the 340 cipher was released on November 16th 1970 with the address of "S.F. Chronicle San Francisco Calif Please Rush to Editor". The author of the Exorcist letter (if a hoaxer) when addressing their envelope to the San Francisco Chronicle only had these two envelopes to work with, but copied neither. This mimicry would have been expected of a hoaxer.
Instead, the author of the Exorcist letter chose to mimic part of the address from two envelopes mailed by the Zodiac Killer in April 1970. These two envelopes were addressed San Fran (dot) Chronicle on April 20th and April 28th 1970 and never publicly released via the newspapers prior to January 29th 1974. When you look at the April 28th 1970 envelope in particular, it isn't just the handwriting that is extremely similar, it's the positioning of the detached full-stop, the spacing between each of the three components, the very slight left to right slant, the marginally elevated letter L, the letter I is dotted directly above in every instance, and both the letters R and H are slightly separated. The April 20th and April 28th envelopes were the only two Zodiac communications up to January 29th 1974 that were addressed in this fashion, yet the author of the Exorcist letter (if a hoaxer) would have managed to virtually replicate every facet of this address despite having never laid eyes on them. This clearly shows that the Exorcist letter author was the Zodiac Killer, putting to rest any notion that this letter may have been fraudulent.
People will say this just further proves that the Exorcist letter was penned by Inspector David Toschi, who had inside knowledge of the two April 1970 letters. Why would Inspector David Toschi copy the address so perfectly from an envelope that had never been released into the public domain by January 29th 1974, thereby drawing suspicion towards somebody from within? He was an intelligent seasoned investigator, not an imbecile. The exact same reasoning can be applied to the 1978 letter when comparing the opening line from each communication. A copycat or investigator would apply logic, and mimic the widely publicized "This is the Zodiac speaking" introduction displayed in four San Francisco Chronicle publications (or mimic the 11 out of the 12 communications prior to January 29th 1974 that carried this introduction), not mimic an introduction never released into the public domain such as "This is the Zodiac speaking I" in the Melvin Belli letter. However, the Zodiac Killer is perfectly allowed to be himself.
For black and white issue..