I had a couple of these that had screw studs on the bottom. I was told by an antique dealer that it was for a bone letter opener that was once attached and that the hole in the top was for a stanhope. I sold them off years ago, but since then I found that it looks exactly like a 17th century flea trap. I wonder if the letter opener was something that was a dual purpose thing. A letter opener and flea trap that could be detached and worn around the neck. I tried to look up the history of these but can find very little. Thanks for any help.