It is the Morgul Knife which vanishes in FotR, in RotK it is Merry's barrow-blade.
A conflation of the 2 could be the source of confusion.
I imagine that at some point when Sting had become Sam's, Sam gave his Barrow sword to Merry. But his is nowhere stated.
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The dwindling Men of the West would often sit up late into the night exchanging lore & wisdom such as they still possessed that they should not fall back into the mean estate of those who never knew or indeed rebelled against the Light.
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