There is an excellent thread discussing this Christmas card on the Zodiac Killer Site forum, but here we would like to approach the 'Eureka' card from a more simplistic angle and the reasoning behind the xeroxed or photocopied keys contained within the communication.
The wording on the card exterior read; "FROM YOUR SECRET PAL CAN'T GUESS WHO I AM YET? WELL, LOOK INSIDE AND YOU'LL FIND OUT..." and once opened it revealed: "...THAT I'M GONNA KEEP YOU GUESSIN'! HAPPY HOLIDAYS, ANYWAY." The suggestion, is that once the card is opened it will reveal the identity of its author- and although there is no written message within the Christmas card- there are two photocopied keys. One would therefore like to believe, that this is the clue to the author's identity in the form of an image. Here is a manufactured image of how the presentation may have looked before a negative was produced.
A Unazod article entitled 'A New Zodiac Missive' shows the writing on the envelope of the 'Eureka' card in comparison to a letter mailed to Mrs Irene Preston from Ted Kaczynski in the December of 1990. It is a compelling comparison.
However, is there a more simplistic answer to the identity of the author using the image mailed to the San Francisco Chronicle.
"Theodore John Kaczynski born May 22, 1942), also known as the Unabomber, is an American domestic terrorist. A mathematics prodigy, he abandoned an academic career in 1969 to pursue a primitive lifestyle. Then between 1978 and 1995 he killed three people, and injured 23 others, in a nationwide bombing campaign targeting those involved with modern technology, in an attempt to start a revolution. In conjunction, he issued a social critique opposing industrialization and advancing a nature-centered form of anarchism.
The initial 1978 bombing was followed by bombs sent to airline officials, and in 1979 a bomb was placed in the cargo hold of American Airlines Flight 444, a Boeing 727 flying from Chicago to Washington, D.C. A faulty timing mechanism prevented the bomb from exploding, but it released smoke, which forced an emergency landing. Authorities said it had enough power to "obliterate the plane" had it exploded. As bombing an airliner is a federal crime, the Federal Bureau of Investigation became involved in the case, designating it UNABOM for UNiversity and Airline BOMber. (U.S. Postal Inspectors, who initially had the case, labeled the suspect the "Junkyard Bomber" because of the material used to make the mail bombs. In 1979, an FBI-led task force that included the ATF and U.S. Postal Inspection Service was formed. The task force grew to more than 150 full-time personnel, but minute analysis of recovered components of the bombs and the investigation into the lives of the victims proved of little use in identifying the suspect, who built his bombs primarily from scrap materials available almost anywhere. The victims, investigators later learned, were chosen irregularly from library research." Wikipedia.
Could Ted Kaczynski have used a photocopying machine during this library research and mailed the 'Eureka' card to the San Francisco Chronicle in the December of 1990? Ted Kaczynski was later to send carbon copies of his 'Manifesto' to various publications: "Last June and July, the Unabomber mailed carbon copies of the 62-page, single-spaced manifesto to the New York Times, Washington Post, Penthouse magazine and Tom Tyler, a UC Berkeley psychology professor. Tyler appears to have received his copy as something of an afterthought by the bomber. His only known connection to the case is that he had been quoted in a Bay Area newspaper commenting on the behavior of the serial killer." Los Angeles Times.
The 'Unabomber' Ted Kaczynski would either mail his explosive devices using the postal service or hand deliver- so were the USPS (United States Postal Service) keys xeroxed and mailed with the 'Eureka' card to the San Francisco Chronicle, as a form of taunting towards the authorities he so obviously railed against in a game of cat and mouse - and within the image a clue to his name using only two alphabetical letters. The etched writing on the keys of USPS DO NOT DUPLICATE could be considered ironic with regards to the xeroxed image supplied.
Earlier in Ted Kaczynski's bombing campaign, on June 10th 1980, Percy Wood, president of American Airlines opened a package containing a book called 'Ice Brothers' packed with explosives, from which he narrowly cheated death. The device was constructed using wood - the victim's last name was Wood and the return address was from a street named Ravenswood. Was Ted Kaczynski playing a game of words with investigators in his attempted murder of Percy Wood? Inside the chiseled out section of the book Ted Kaczynski had inserted a small piece of metal in which the letters "FC" had been punched into its face. These two alphabetical letters would be used throughout his bombing campaign to identify his work to authorities. "In all, 16 bombs—which injured 23 people and killed 3—were attributed to Kaczynski. While the devices varied widely through the years, all but the first few contained the initials "FC." Inside his bombs, certain parts carried the inscription "FC," which Kaczynski later asserted stood for "Freedom Club."
One of Kaczynski's tactics was leaving false clues in every bomb. He would deliberately make them hard to find to mislead investigators into thinking they had a clue. The first clue was a metal plate stamped with the initials "FC" hidden somewhere (usually in the pipe end cap) in every bomb. One false clue he left was a note in a bomb that did not detonate, which reads "Wu—It works! I told you it would—RV." Wikipedia.
One can see that Ted Kaczynski was fond of leaving cryptic clues for investigators to find - so is it any great leap of faith that Ted Kaczynski would choose a Christmas card beginning with the lines "From your secret pal. Can't guess who I am yet? Well look inside and you'll find out." So If Ted Kaczynski was to leave his name within the 'Eureka' card, then it had to be connected with the xeroxed image, and quite possibly may have been two initials in length.
The image presented is that of 'Two Keys,' the initials of which are TK, representing Ted Kaczynski.
It's a simplistic analysis - but it's a simplistic image, with limited room for maneuver.