It's from the early 20th century era, it was a device you put on your keyring to help prevent you from losing your keys.
As boating became more popular on the lake, people realized that while they rarely misplaced their keys, the device was not very buoyant (though it was able to be easily retrieved from the water with a 30lb magnet) and so they died in popularity.
If you brush away some of the rust you may still be able to see an inscription like "Niagara Falls, New York" or "I (heart) Winnipesaukee", as they were popular tourist souvenir items.
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