In 70 AD, Nero was, I believe for the first time in history, deliberately debasing currency through coin clipping. His mark on the coins was required for them to be considered money, and unlike before, was the real determinant over what it was worth, rather than the weight of the metal it was made of.

And yea, if you're not familiar, 666 refers to Nero by way of Gematria, a form of qabalistic numerology. Specifically, to Neron Caesar as it would have been rendered in Hebrew.

Currency debasement leads to a predictable set of consequences, which look very similar across history, even if the specific technology that gets used in  each episode varies a bit.