Then the shorter piece of reed was artificially hollowed to fit on the longer piece.

Apparently it formed a protective sheath or cap.

But what did it protect?
Presumably a point or edge of some kind.

Could this be a pocket-knife of any sort, such as a small stencil-knife?
No; the material was too fragile for a knife-handle.

It could not be an etching-needle for the same reason; and it was not a surgical appliance--at least it was not like any surgical instrument that was known to me.
I turned it over and over and cudgelled my brains; and then I had a brilliant idea.