A lost libretto by Mahler

At Sotheby’s music sale next month, they are putting up an apparently unknown first draft of Das Klagende Lied, the composer’s first attempt to create a Singspiel.

Here’s what the catalogue says about it:

Autograph manuscript of the poem “Ballade vom blonden und braunen Rittersmann”, AN EARLY, HITHERTO LOST, DRAFT FOR THE TEXT OF THE FIRST VERSION OF DAS KLAGENDE LIED, signed (“Gustav Mahler”) comprising three parts, numbered by the composer “I”-“III”, of nine, five and seven stanzas, respectively, with autograph title (“Ballade vom blonden und braunen Rittersmann”), signed and dated at the end (“Gustav Mahler 18.1-3/379.”)
…Vom Felsen hoch erglänzet das Schloß,
die Zinken erschall’n und drommetten,
dort sitzen der Ritter muthiger Troß,
und die Frauen mit goldenen Ketten!
“Was will der jubelnde, fröhliche Schall,
was leuchtet und glänzet im Königssaal
o Freude, heia Freude!”…
8 pages, large 8vo (22.5 x 14.3cm), the second and third
pages paginated apparently by the composer, ink-blot to lower
margin of first page, later pencil annotation to last page (“7569
inv.”), [Vienna,] 1-3 March 1879, horizontal and vertical folds,
some splitting to hinge and folds
A SIGNIFICANT MAHLER DISCOVERY, ADDING TO OUR
KNOWLEDGE OF THE DEVELOPMENT OF THE TEXT FOR THE
FIRST VERSION OF DAS KLAGENDE LIED.
THE TEXT OF THIS MANUSCRIPT IS SUBSTANTIALLY
UNPUBLISHED, THE LAST TWO SECTIONS BEING UNKNOWN
TO SCHOLARS.