President John F. Kennedy was assassinated on November 22nd 1963 in Dallas as his motorcade traveled along Dealey Plaza. The rifle used to kill President Kennedy was purchased by mail-order via an advertisement in the monthly magazine, American Rifleman. This generated widespread debate and congressional hearings for the need to ban mail-order sales of firearms. However, the bill was not passed for another five years. The assassination of Martin Luther King on April 4th 1968 and the murder of Robert F. Kennedy on June 5th 1968, accelerated the desire for more gun control in the United States. The Gun Control Act was signed into law by President Lyndon B. Johnson on October 22nd 1968, banning mail-order sales of rifles and shotguns and prohibiting most felons, drug users and people found mentally incompetent from buying guns. The Zodiac Killer was fully aware of the approaching clampdown on gun sales, when on November 9th 1969 he stated "my killing tools have been boughten through the mail order outfits before the ban went into efect. Except one & it was bought out of the state". If we believe his claim, then he likely purchased the majority of his weapons in California prior to October 22nd 1968, possibly in anticipation of the carnage he was about inflict on the Bay Area residents.
Bearing in mind the Zodiac Killer may have been a fisherman, hiker or hunter, with a penchant for killing humans because "it is more fun than killing wild game in the forest", then it's not inconceivable that he purchased his weapons through this magazine before the ban went into effect. This obviously wasn't the only gun magazine on the market, but it was this magazine that triggered the debate on gun control in the first place. The pencil flashlight, he claimed was taped to the barrel of his gun at Lake Herman Road, could have been inspired by an article featured in American Rifleman or similar magazine. The gun magazines released in the months prior to December 20th 1968 might be a valuable resource in learning a little more about the Zodiac Killer.