The Scofield Bible Commentary, by Cyrus Ingerson Scofield, [1917], at sacred-texts.com
Keturah
As Sarah stands for "the mother of us all," that is, of those who, by grace, are one with the true Son of promise, of whom Isaac was the type (Joh 3:6-8); (Gal 4:26); (Gal 4:28); (Gal 4:29); (Heb 2:11-13) and joint heirs of His wealth; (Heb 1:2); (Rom 8:16); (Rom 8:17); so Keturah (wedded after the full blessing of Isaac) and her children by Abraham may well stand for the fertility of Israel the natural seed, Jehovah's wife (Hos 2:1-23) after the future national restoration under the Palestinian covenant.
(See Scofield) - (Deu 30:3).
Esau
Esau stands for the mere man of the earth. (Heb 12:16); (Heb 12:17). In many respects a nobler man, naturally, than Jacob, he was destitute of faith, and despised the birthright because it was a spiritual thing, of value only as there was faith to apprehend it.
birthright
The "birthright" had three elements:
(1) Until the establishment of the Aaronic priesthood the head of the family exercised priestly rights.
(2) The Abrahamic family held the Edenic promise of the Satan-Bruiser (Gen 3:15). -- Abel, Seth, Shem, Abraham, Isaac, Esau.
(3) Esau, as the firstborn, was in the direct line of the Abrahamic promise of the Earth-Blesser (Gen 12:3).
For all that was revealed, in Esau might have fulfilled those two great Messianic promises. This birthright Esau sold for a momentary fleshly gratification. Jacob's conception of the birthright at that time was, doubtless, carnal and inadequate, but his desire for it evidenced true faith.