Matt,
People have been posting these on Tnet for years and we can't figure them out. Many have a "pry mark" on one edge. They are not buttons but must have been a common throw-away item over the last century or so.
There is a really long thread from a couple years ago that speculates about them, but no real answer to the mystery. Thanks for posting. Maybe it will spur someone to research some more. Several good research have spent a lot of hours on these and basically come up empty. Frustrating to say the least.
DCMatt