"Concerning... 'The Hoard'" revealed

[Tar Elenion]

Originally Posted by Huinesoron

Per gondowe, this seems to be the only source for the death of Maglor.

Letter 131 is another source:
"The remaining two Silmarils are regained from the Iron Crown — only to be lost. The last two sons of Fëanor, compelled by their oath, steal them, and are destroyed by them, casting themselves into the sea, and the pits of the earth. The ship of Earendil adorned with the last Silmaril is set in heaven as the brightest star. So ends The Silmarillion and the tales of the First Age."

and compared with Concerning the Hoard:
"The other two Silmarils were also taken by the Valar from the crown of Morgoth. But the last surviving sons of Feänor (Maedros and Maglor), in a despairing attempt to carry out the Oath, stole them again. But they were tormented by them, and at last they perished each with a jewel: one in a fiery cleft in the earth, and one in the sea.
So the three Silmarils were lost for ever “until the remaking of the world”: in air, earth, and sea. Thus ended the First Age."

Edit:
Going through Letter 131 similar comparisons in verbiage can be drawn, e.g.:
"The chief of the stories of the Silmarillion, and the one most fully treated is the Story of Beren and Lúthien the Elfmaiden."
Letter 131

"The most important “tale” in this [[† network?]] of legend is that of Beren and Lúthien, but as that is sketched in the Lord of the Rings it is not told here."
Concerning...
et.al.