The form of Latin S occurs from Greek Σ by dropping a person out in the 4 strokes of that letter. The (angular) S-form made up of a few strokes existed as being a variant from the 4-stroke letter Σ already while in the epigraphy of Western Greek alphabets, as https://bud-naked-mushrooms21874.thelateblog.com/31842097/not-known-facts-about-bud-naked