It looks Japanese.
Maybe for leather working
I mentioned Japan because the third pic down looks like Kanji or other Japanese characters.I thought I saw USA in one of the pics. I agree with leather working tho.
Based on the Osborne system, I’d guess 5/16”, but not shaped like Osborne’s older ones.
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Osborne Tools : Catalog No. 71 : C. S. Osborne & Co. : Free Download, Borrow, and Streaming : Internet Archive
72 p., illus., 27.9 cm, trade catalogarchive.org
Most of mine are Osborne, so I might not be much help, will look tomorrow.