Good answers!
Perhaps there was a "Museum of Westernesse" in Minas Tirith, or even more likely one in the vastly-ancient Pelargir.
It may also be that while Arnor was at war within itself and with Angmar, some Dunedain families may have emigrated south to the relative "safety" of Gondor, bringing their crafts with them.
The idea of anti-Angmar runes in Adunaic legible to Denethor seems the most likely explanation for his recognition of the blade.
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