S NO FURTHER A MYSTERY

The form of Latin S arises from Greek Σ by dropping just one out of your 4 strokes of that letter. The (angular) S-shape made up of 3 strokes existed like a variant from the four-stroke letter Σ already during the epigraphy of Western Greek alphabets, plus the a few and 4 strokes variants existed alongside each other while in the classical Etrusc