Traditional Japanese knife edge shaping tool called sen.
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Previously:
https://www.homemadetools.net/forum/...473#post122635
https://www.homemadetools.net/forum/...212#post176743
https://www.homemadetools.net/forum/...609#post178162
https://www.homemadetools.net/forum/...473#post154865
https://www.homemadetools.net/forum/...486#post177753
https://www.homemadetools.net/forum/...788#post136936