It never ceases to amaze me what you can find on the internet. Now, I'm no expert on beavers, but I don't think 19th century plow planes are part of their diet, and if they were, this one would probably have to have been found in a swamp. Or maybe the craftsman of the day had a pet beaver? Does beaver damage add to the value?
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In all seriousness this looks like maybe someone's dog decided to use it as a chew stick. Or one of the wooden screws broke of the end and they hacked away at it so they could still unscrew the knob. Maybe I'm wrong, but I highly doubt it's beaver damage.
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