The object in the photo does look like a grenade pin. I have seen many of these items and as a matter of fact I have a few of them! The split ring and pin are commonly used on some camping tents to secure the tent pole to the tent/fly body. Nice pile of goodies you found. Its a great feeling and a rush when our detectors makes a "hit" in the sand. As you know, while metal detecting on open beach, we sometimes go for long periods of time while searching. Its always good to hear that beep!