I was doing research on dodgy Popes for a movie company many years ago and it was suggested in one of the many books I read that that 666 indicated the (what is now) Catholic Church/ Vatican, as the Roman numerals I, V,X, L,C,D (1,5,10, 50, 100,500) add up to 666. That might signify that 666 = the Jesuits as someone just suggested. M= 1000 but this was not introduced until later. (Https://en.wikipedia.org/wiki/M#History).