Actually, I believe the "S" would be the mint, in this case Seville, Spain. As ofr being authentic or not, I would have to say NOT, by the picture. The markings on the shield look more "cast" then stamped. Edges are too soft, rounded and have almost a "flattened" appearance.
Sorry, but I'd have to say "pass" on this one if it was offered at a show/shop.