I posted this on the ID forum, and was referred here. There seems to be some question on authenticity, which I have to admit I question also. This was found on a North Carolina beach on Monday.
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.