first us military button

I took my little son and wife to a state (mountain)park. I was going to check a strange pile of flat rocks at the top which I suspect are man made but instead right at the entrance to a trail is a fenced in old foundation,so I checked the front and dug a copper button with an eagle holding an anchor and three cannon balls at the bottom. So its Navy. It is plain(no stars) I think its from the 1940s? I found exact one on ebay. I will need to go back because my son went crazy on me and my wife thought she had her ipod stolen right there!