The author of the April 24th 1978 "I am back with you" letter (if a hoaxer) had every opportunity to just mimic any one of these common introductions, yet he chose to mimic the Melvin Belli letter on December 20th 1969, which broke with tradition. The Melvin Belli message began with "This is the Zodiac speaking I", but was grammatically incorrect in failing to place a comma or full-stop between "speaking" and "I". Therefore, the author of the 1978 letter (if a hoaxer) chose to imitate the message on the opening line of the Melvin Belli letter (including the punctuation error) rather than the standard introduction used by the Zodiac Killer.
Excerpt from Wikipedia on And the Children Shall Lead: "The federation starship Enterprise arrives at the planet Triacus. Captain Kirk, Dr. McCoy, and First Officer Spock beam down in time to witness the death of Professor Starnes, the leader of a scientific expedition team. The other members of the expedition, apart from their five seemingly unconcerned children, seem to have died at their own hands. The crew bring the children back to the Enterprise, where McCoy evaluates them and determines that they are suffering from lacunar amnesia, unaware of what happened to their parents and unable to grieve. However, when left unattended in one of the ship's rooms, the children chant an evocation and summon a glowing humanoid named Gorgan. He advises them to take control of the crew in order to get to Marcus XII, his preferred destination. The eldest child, Tommy, uses mental powers Gorgan has bestowed on the children to trick the crew into steering the ship while presenting illusions that make them think they are still in orbit above Triacus. Upon reviewing a troubling expedition film recorded by Starnes, Spock, McCoy, and Kirk return to the bridge to find the children and Gorgan fully in control of the crew. Unable to break their hold on the crew, Spock observes that the children are merely possessed by Gorgan, who must be the evil embodiment of an ancient group of space-warring marauders released by Starnes's archaeological survey".
1978 letter: "I am now in control of all things".
THE "I AM BACK" PHONE CALL - MARCH 13TH 1978