666 - the number of a man - was a reference to Sextus (which means 'sixth son') Hermetidius Campanus: a legatus (commander of a legion) in Judea during the late AD 90's.
The mark of the beast was the imprimature of the emperor found on coins and legal documents (both of which you needed to buy or sell).
Legions serving over seas were also permitted to 'counter-strike' coins - that had been worn smooth by use - with their own emblems. The Tenth Fretensis: the legion that served in Judea throughout the first century AD, counter-struck coins with a stylized pig (boar).
After the destruction of Jerusalem in AD 70 most of the initial re-building was undertaken by Roman soldiers. So the legionary emblems also got stamped into bricks and floor tiles. Any Jew visiting Jerusalem during this period would have seen the mark of the beast everywhere he looked.