Stratros 
I found the following extracts hereGreek Alphabetic Numbers at the Time of the New TestamentSince the Book of Revelation, like the other twenty six
books of the New Testament, was written in Koine Greek in the first or early second century AD, we must study how numbers were represented at that time in Greek.[5]
By the time of the New Testament we need only consider the Greek alphabetic numeral system.  Although the Greeks had ways of representing numbers in the thousands and also those above ten-thousand (myriads), we will only consider how they represented numbers up to nine hundred ninety nine.
These Greeks, like us, used a decimal system but, unlike us,
they did not use a zero number[9].  Thus, although it was a
decimal system, it was not a place-value system.  Therefore more symbols were needed than just the nine (plus zero) which we use. To represent the numbers up to our 999, they needed 27 different symbols:  nine for the units 1, 2, ..., 9; nine for the tens 10, 20, ..., 90; and still another nine for the hundreds 100, 200, ..., 900.
For symbols they used the letters of the Greek alphabet.
However, an immediate problem arose since the Greek alphabet contained only 24 letters.  To fill in the missing three, they used old obsolete letters from previous alphabets and, somewhat confusingly, these were interspersed with the regular letters.
The following table show the assignments[10].
    TABLE OF NUMERICAL VALUES OF GREEK LETTERS
     1 alpha        10 iota       100 rho
     2 beta         20 kappa      200 sigma
     3 gamma        30 lambda     300 tau
     4 delta        40 mu         400 upsilon
     5 epsilon      50 nu         500 phi
     6 [digamma]    60 xi         600 chi
     7 zeta         70 omicron    700 psi
     8 eta          80 pi         800 omega
     9 theta        90 [koppa]    900 [sampi]
The digamma at 6, however, will be directly relevant for the
representation of six hundred sixty-six.  This letter also became known as stigma (note the "t") because the form of writing it looked like the ligature (writing together) of the two Greek letters sigma and tau, roughly corresponding to our s and t.  [If not done carefully, it can even look like just a sigma, which would be an error.]  For most of the rest of this study this obsolete letter pushed into service to represent the numeral for 6 will be referred to as stigma.
Thus to represent our number of interest, six hundred
sixty-six, rather than using three identical characters (666) as we do in our decimal PLACE-VALUE system, a first century Greek writer (if not spelling out the words just as we also can) would use three distinct characters (chi)(xi)(stigma)!
To summarise Stratos: the book of Book of Revelation, written in Koine Greek in the first or early second century A.D.
If they wanted to use Greek numerals to write 666, they would've used the numerals chi xi stigma.
chi xi stigma looks like: the crossed swords of islam and the bismallah on it's side.
As I've said before
600 60 6 = 666
or
  6
 60
600    = 666