The New Testament quotes portions of the Greek Old Testament (known as the Septuagint, or LXX for short), including portions of books beyond the Pentateuch (or five books of Moses), some of which are of a distinct tradition than that of the Hebrew (or what is loosely called the Masoretic tradition).1
Yet it is claimed by Jews,2 as well as others, that the Septuagint was originally only a translation of the five books of Moses, not the entire Bible.
Question: If this is so, what exactly were the New Testament authors quoting?
Thanks in advance.
1 e.g. Mt 9:13 "mercy" (LXX) / cf. Hos 9:6 "goodness" (Masor); Mt 9:13 "perfected praise" (LXX) / cf. Ps 8:2 "ordained strength" (Masor). See also Mk 7:6-8/Isa 29:13; Lk 3:5-6/Isa 40:4-5; Rom 2:24/Isa 52:5 | It's been estimated that out of all the quotations of the Old Testament in the New, 340 are from the LXX, and 33 from the Masoretic (see G. Archer and G. C. Chirichigno, Old Testament Quotations in the New Testament: A Complete Survey, p. 25-32).
2 The 1906 Jewish Encyclopedia (Bible Translations, under 'Septuagint') states, "According to Aristeas, the Pentateuch was translated at the time of Philadelphus, the second Ptolemy (285-247 B.C.), which translation was encouraged by the king and welcomed by the Jews of Alexandria. Grätz ("Gesch. der Juden," 3d ed., iii. 615) stands alone in assigning it to the reign of Philometor (181-146 B.C.). Whatever share the king may have had in the work, it evidently satisfied a pressing need felt by the Jewish community, among whom a knowledge of Hebrew was rapidly waning before the demands of every-day life. ¶ It is not known when the other books of the Bible were rendered into Greek. The grandson of Ben Sira (132 B.C.), in the prologue to his translation of his grandfather's work, speaks of the "Law, Prophets, and the rest of the books" as being already current in his day. A Greek Chronicles is mentioned by Eupolemus (middle of second century B.C.); Aristeas, the historian, quotes Job; a foot-note to the Greek Esther seems to show that that book was in circulation before the end of the second century B.C.; and the Septuagint Psalter is quoted in I Macc. vii. 17. It is therefore more than probable that the whole of the Bible was translated into Greek before the beginning of the Christian era (Swete, "An Introduction to the O. T. in Greek," ch. i.). The large number of Greek-speaking Jewish communities in Palestine, Syria, Mesopotamia, Asia Minor, and northern Africa must have facilitated its spread in all these regions. The quotations from the Old Testament found in the New are in the main taken from the Septuagint; and even where the citation is indirect the influence of this version is clearly seen. This will also explain in a measure the undoubted influence of the Septuagint upon the Syriac translation called the "Peshiṭta.""