I guess back then they only had 3 digit PIN codes.
LOL, Back then, they added the digits together, i.e., 6+6+6 = 18 and 1+8 = 9, the number of digits in our SSN.
One possible interpretation of “666” is this.
The “number of Man”, Biblically, is usually considered to be “6”, since Man was created on the Sixth Day.
Three is the number of God - the Trinity.
Thus, “666” is the number of a Man making himself out to be god, i.e, “Worship Me!” “My word is Law!” “I will give you salvation.”
This is one interpretation I’ve heard, and it seems valid. But, as the Book says, “This calls for wisdom. Let the person who has insight calculate the number of the beast, for it is the number of a man. That number is 666.” Rev 13:18.