The form of Latin S occurs from Greek Σ by dropping just one out of your 4 strokes of that letter. The (angular) S-condition made up of a few strokes existed for a variant of your 4-stroke letter Σ now in the epigraphy of Western Greek alphabets, as well as https://budnakedmushrooms31975.blogtov.com/11777803/the-smart-trick-of-bud-naked-exotic-that-no-one-is-discussing