First of all Donald Lee Bujok had a military background, serving with the Sixth Infantry for approximately eight months in 1954, and while stationed there would have come across the California Veterans Association symbol which looks very similar to the symbol the Zodiac placed on the upper left of the envelope and at the foot of the card once opened (seen here below left).
The author of the 'Halloween' card envelope also underlined the L, A and V of the mispellt name of Paul Avery, indicating he was a member of the Light Armored Vehicle Division, with the misspelling to Averley hinting towards his middle name of Lee.
The front of the 'Halloween' card contains the message, "I feel it in my bones, you ache to know my name, And so I'll clue you in," and once the card is opened it reveals the word BOO!, signifying his name BUJOK. The card was also 'sprayed' with multiple eyes, drawing the observer to another subtle hint left by Donald Lee Bujok, that he was a member of the Sightseeing Sixth Infantry Division.
Donald Lee Bujok was stationed at Fort Ord in the East Garrison close to where this emblem was located in the bottom left corner of the image.
The skeleton positioning in the card was possibly portraying memories of his time in prison, in which prisoners were hung by their wrists to the cell bars, as a form of punishment, and similar to how Paul Stine's body was discovered in the taxicab at the Presidio Heights murder scene.
The writing By Gun, By Knife, By Rope and By Fire represented methods employed by prisoners to taunt guards they had held captive during an uprising at the prison on 16th April 1959, which became widely known as Jerry's Riot. Another prisoner Ed Edwards, who had served alongside Bujok, later released a book called Metamorphosis of a criminal. This book described another inmate, who Ed Edwards recalled as his most uncomfortable memory from his time at the Deer Lodge Prison. A prisoner who held the belief that anyone being killed by him would become his slave in the afterlife. This is possibly where the inception of the Zodiac Killer, along with his inspiration gathered momentum, eventually to be realized during later communications, such as the July 31st 1969 408 cipher and 'Little List letter' mailed on July 26th 1970.
Finally photographs of Donald Lee Bujok wearing military issue glasses, which he often wore in later life, bear an uncanny resemblance to the famous Zodiac composite sketch.
For the full analysis of Kevin Robert Brooks breakdown and theory, on why Donald Lee Bujok is a firm candidate for the Zodiac attacks, please visit here: http://www.youtube.com/watch
He is now waiting on news from the FBI and Napa County for results on the evidence he has submitted to see if his prime suspect matches DNA and fingerprints held in storage in the Zodiac case files.