Convex button with a weird shank.

Found a weird button today at the spot where I found a musket ball and an 1805 large cent. It was 8 inches down, so I'm thinking that it's pretty old. It's convex and measures 1 1/8" in diameter. Probably made out of brass, but the shank looks like twisted iron. I'm wondering how old it may be and if the shank is original or a quick fix for the broken shank. Thanks for any help on this one.