Sacra Moneta Kyzicus Sexta, that is Sacred Money, struck at Kyzicus mint by the sixth officina (or workshop).
Actually S in this mintmark does not stand for Sexta, though it is the sixth workshop... it is just a coincidence. Here is a reply from another post I made about this -- "The last symbol in your mintmark looks like an "S" but is actually ϛ (digamma); which is Greek for the sixth workshop. Everyone calls it an S, myself included..."
Victor is certainly right about the digamma from Cyzicus but we must toss in this coin of Trier with TRS mintmark where the S indicates secunda or 2nd shop. We can't tell everything from one coin. Trier, at the time, was using a two shop system Prima and Secunda while Cyzicus has six lettered by Greek numerals one through six or A through S. The point is you have to look at the big picture before we can make the call correctly. So, is there an example where S does mean sexta? Some mints dodged the problem of Secunda and Sexta both starting with an S by using P S T Q V and VI (V for five was needed because the Q of quinta and quarta problem). The one I can't call is Rome which for a while used P B T Q E and S. P, T and Q were ordinals in Latin while the B and E were Greek numerals. Was the S Sexta or the digamma? I don't have a Rome S meaning six. Want list item? What is the RS coin below? No, it is a shop two because when it was issued Rome only used five shops so S was Secunda=2 in a PSTQE system. The thought process when they opened the sixth shop and moved to B=2 is wholly beyond me. You gotta luv those Romans!
Digamma, waw, or wau (uppercase: Ϝ, lowercase: ϝ, numeral: ϛ) is an archaic letter of the Greek alphabet. https://en.wikipedia.org/wiki/Digamma
We're both correct. "In modern practice, the term stigma is often applied to the symbol ϛ both in its function as a ligature and as a numeral, whereas the term digamma is normally used for the ancient letter representing /w/, which appears in modern print as Ϝ or ϝ (the form has a large number of close variants)." https://en.wikipedia.org/wiki/Stigma_(letter)
If you want, you can follow the development of the numeral on coins. Look at this one from a century earlier (Philip I).