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That is a lovely Mappin & Webb sportsman’s knife. I have never seen anything like the apparatus you show. If I understand correctly, some (presumably round) object having a diameter no larger than the opening in the scale is inserted into the opening, The object is then struck forcefully on its end from the side.
The only thing which occurs to me, after much thought, is that it is intended to break a small capsule of smelling salts
They were a popular remedy in Victorian times, both for ladies afflicted with “the vapours” and athletes suffering from exhaustion or mild concussions.
I’ll be interested to hear other ideas. Thanks for sharing a most interesting and unusual knife.