The shape of Latin S arises from Greek Σ by dropping just one out of the 4 strokes of that letter. The (angular) S-condition made up of a few strokes existed to be a variant of your 4-stroke letter Σ now in the epigraphy of Western Greek alphabets, as well https://budnakedmushrooms53197.blogunteer.com/30531636/s-no-further-a-mystery