Antique metal button

This item has no history except that it was in an envelope labeled "Antique Button."
It feels heavy for its small size, so I assume it is metal.
Very difficult to see, even close up with good light, but there appears to be a brass-colored inlay in the darker metal, but no discernible shape to it. The "V" shape is a fairly deep depression in the metal.
On the back side there is a very thin raised edge.
1/2" diameter x 1/4" high (including shank)

Any pointers much appreciated!