A fine 18th century bistoury cache (literally hidden knife), first used in the mid 18th century for lithotomy (cutting into the bladder to remove stones).
The asymmetrical polyfaceted ebony handle when rotated would variably limit the extension of the blade. Each face of the handle is numbered. This instrument as shown in Bennion p 64 is the same as Frère Côme’s Bistoury held in the Royal College of Surgeons. There is a similar instrument in display in the Wellcome Museum.