S Model
There was a practice of naming automobile models starting with A thru Z in the late 1800s and early 1900s (thru about 1936). Also numeric sequences; 100,200, 300... that hold true today.
We have a 1931 A Model Ford otherwise known as a Model A Ford. There were two A Model Ford generations made; 1903 and 1928-31.
It would not surprise me to know that Tesla has been following this pattern in modern times ( S, X, 3?)