The form of Latin S arises from Greek Σ by dropping just one out of your 4 strokes of that letter. The (angular) S-condition made up of 3 strokes existed for a variant of your 4-stroke letter Σ by now during the epigraphy of Western Greek alphabets, as well as https://budnakedflower22075.bloguetechno.com/s-no-further-a-mystery-66605466