Why do i always assume that, if a blade has a ridge (even if the other face is flat) has to be that of a weapon and not of a domestic knife ?

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I picked this one for less than a price of a portion of fish and ships (as per Tim Simmons

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It came with a modern turned wood handle, which i torn off within the hour.
The tang is no so 'martial' but ...
I wonder what the hole represents, or what is its purpose

. Anyone familiar with such feature ?
A nice patina is present, a possible sign of its age as being 19th. century ?
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