Gerald Tagert had a 7-year-old son in 1970 (Greg Tagert), who went on to become a professional baseball coach. There is a high probability that if these tickets were mailed by the Zodiac Killer, he knew Gerald Tagert well enough to know he had a young son obsessed with baseball, who was a potential recipient of the second baseball ticket (Gerald had two other sons, Dave and Chris). The two Oakland A's tickets were likely a "gift" for "Father's Day", which was on Sunday, June 21st 1970 (the day after the date on the Oakland A's tickets). A "Gift from Zodiac" for father and son, wrapped up in "wrapping paper" like a present. If these tickets were mailed to Gerald Tagert exclusively with "Father's Day" in mind, then the fact that Chicago While Sox were the opponents that day had no relevance to the sender of the letter. The tickets were presented as a gift addressed to Gerald Tagert rather than Mr and Mrs Tagert.
If Gerald and Greg Tagert were the intended recipients on June 19th 1970, then the act of offering a baseball ticket to a 7-year-old child with the accompanying signature of "Zodiac" should undoubtedly be perceived as sinister, bearing in mind the Zodiac Killer had previously threatened school children on more than one occasion. At the very least, the sender of this letter knew the name of the householder, understood it wasn't a single occupancy residence, and was fully aware that the occupants had an interest in the Oakland A's baseball team. They may also have been aware that Saturday, June 20th 1970 was "Helmet Day" when 25,000 gold batting helmets were being given away free to all youngsters 14 years of age or under (see newspaper cuttings below). This was the first "Helmet Day" in the club's history.
The Athletics Major League Baseball franchise began in Philadelphia before moving to Kansas City in 1955 and then to its home at the Oakland Coliseum in 1968. The Oakland A's made their Bay Area debut on Wednesday, April 17, 1968, with a 4–1 loss to the Baltimore Orioles in front of an opening-night crowd of 50,164. Greg Tagert's earliest memories of baseball were evenings spent at the Oakland Coliseum.
FURTHER READING: A POSSIBLE AUTHOR OF THE NOVATO LETTER?