Transmission theory: what the heck was Tolkien thinking?

[Zigûr]

Originally Posted by Kuruharan

unless the records Bilbo worked from were saved archives from Arnor, which I suppose is possible.

I always assumed that this would be the solution. It works perfectly, doesn't it? Elrond kept other artefacts of Númenor in Rivendell, and "Translations from the Elvish" doesn't necessarily mean "Translations of works written by Elves", just "Translations of works written in an Elvish language", which the Númenórean literature in Arnor presumably would have been.