In 1981 Mary Stewart Relfe published "The New Money System: The 666 System is Here". Like in Willard Cantelon's earlier "The Day the Dollar Dies", and in countless conspiracy theories since, Relfe identified cashless transactions and credit cards with the Mark of the Beast. In a more novel vein, she pointed out that if you add the (then-new) zip-plus-four codes to a person's nine-digit social security number, you get a unique identifier for all Americans in 18 digits - a triad of sixes, suggesting 666.
(Never mind that Social Security numbers are already unique among American citizens, making the addition of the zip code superfluous.)
