Firstly, Donald Lee Bujok had a military background, serving with the Sixth Infantry Division for approximately eight months in 1954. The 6th Infantry Division of the United States Army was active in World War I, World War II, and the last years of the Cold War. Known as "Red Star", and formerly called the "Sight Seein' Sixth." The 6th Division was reactivated on the 4th October 1950 at Fort Ord, California. There the division remained throughout the Korean War, training troops and providing personnel for combat, but was never deployed overseas as an entity itself and was again inactivated on the 3rd April 1956.
While stationed there, Bujok 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).
Classifications are detailed in Wikipedia: "Registrant not acceptable for military service. To be eligible for Class 4-F, a registrant must have been found not qualified for service in the Armed Forces by a Military Entrance Processing Station (MEPS) under the established physical, mental, or moral standards. The standards of physical fitness that would be used in a future draft would come from AR 40-501". link
Also the familiar Zodiac crosshair symbol could be found at the garrison during the time Bujok was stationed there (see below right). It marked the spot for a helicopter landing pad, and this is where he formulated the inspiration that he used in later Zodiac correspondence.
The author of the 'Halloween' card envelope also underlined the L, A and V of the misspelled name of Paul Avery, indicating he was a member of the Light Armored Vehicle Division, with the misspelling to Averly 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.
A prison disturbance broke out at Old Montana State Prison in 1959, detailed in an excellent book by Kevin S. Giles. In it he described the punishment meted out by prison guards at the jail, of "a cruelty that included extortion and beatings, that included blackjacks and iron pipes. Parolees came forward with tales of some convicts being tear gassed in their cells. Others hung Christ like from the bars in the hole, where handcuffs chaffed their wrists until they bled. In this dungeon men lived round the clock on bread and water diets. Stories got out that in some cases they got a surprise meal of pork, purposefully soaked in salt. Thirsty men cried out for more than two cups of tepid water that was allowed to them." link
On July 26th 1970, the Zodiac Killer would mail the 'Little List' letter, in which he described what he would like to do to his slaves in the afterlife: "Others shall be placed in cages + fed salt beef untill they are gorged then I shall listen to their pleass for water and I shall laugh at them." Were these images from Old Montana Prison seared into the mind of Donald Lee Bujok, just like the following phrases on the 'Halloween' card.
The writing By Gun, By Knife, By Rope and By Fire represented methods employed by prisoners to taunt guards they had held captive during the uprising at the prison on 16th April 1959, which became widely known as Jerry's Riot. Here is an excerpt from Wikipedia "This started a twenty-four-plus hour standoff in which Myles railed openly to the media outside the walls; Alton retired to his cell after an argument with Myles, convinced that no escape was forthcoming; Jones was again allowed to leave the prison to negotiate with Powell and, under orders from the warden, did not return; and the hostages survived repeated threats of death by fire, rope, or knife. The hostages were eventually crowded into three cells, and the frightened men planned to press the thin prison mattresses against the bars to ward off any attack, but they knew the shield would not hold long against fire or at all against the rifles." link
One prisoner, Edward Wayne Edwards, who served alongside Donald Lee Bujok in Montana Prison, would later go on to release a book entitled Metamorphosis of a criminal. This book detailed the harsh living conditions inside the jail and several of its inmates. One in particular caught the eye of Edward Edwards, who he described as his most 'uncomfortable memory' from his time at the Deer Lodge Prison. A prisoner who held the belief that anyone killed by him would become his slave in the afterlife. Here is the crucial extract:
Unfortunately, this inmate idolized me, for the simple reason that I had earned the respect of the population- something, deep down, he desperately wanted to do. He had an uncanny knack for making your skin crawl. Today, when I think about all the different types I met in that excuse for a penitentiary, this man remains my most uncomfortable memory." PDF page 230
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.