Well the KJV that I use was translated from the Hebrew text and it reads "virgin" in Isaiah 7:14, so that verse proves nothing.
My question is still a legitimate one. What is the pedigree of today's Septuagint from which these verses are quoted? Can the Septuagint of today be traced back to the Septuagint of the first century, particularly where these readings are concerned.
Is it true that all of today's Septuagints are revisions of the Septuagint in Codex B with no earlier pedigree? And how many of these have been revised further with readings from the Dead Sea Scrolls?
Well, obviously that verse was not translated from Hebrew. All Septuagint versions say "virgin" while none of the Hebrew ones do.
And how many of these have been revised further with readings from the Dead Sea Scrolls?
The Dead Sea Scrolls have shown that Judaism did not have only one canon and textual traditions. The oldest LXX codices date back to 3rd century BC and they differ from the Masoretic version.
Al this is irrelevant. The Apostles used the Septuagint as a norm. Obviously they took exception to the LXX as they have to the MT. Either these exceptions are significant in which case neither version is the inerrant word of God, or these variations are irrelevant in which canse either one is as good as the other. The example of Matt 1:23 shows that they are not irrelevant, and our preference in that regard is clear.
4. Reconstruction of Original [Septuagint] Text:
"The task of reconstructing the Oldest text is still unaccomplished. Materials have accumulated, and much preliminary "spade-work" has been done, by Lagarde in particular (see his "axioms" in Swete, Introduction, 484,) and more recently by Nestle and Rahlfs; but the principles which the editor must follow are not yet finally determined. The extent to which "mixture" has affected the documents is the stumbling-block. Clearly no single Moabite Stone presents the oldest text. That of codex B, as in the New Testament, is on the whole the purest. In the 4 books of "Reigns" (1 Samuel through 2 Kings), e.g., it has escaped the grosser interpolations found in most manuscripts, and Rahlfs (Sept.-Studien, I, 1904) regards its text as pre-Origenic. It is, however, of unequal value and by no means an infallible guide; in Judges, e.g., its text is undoubtedly late, no earlier than the 4th century AD, according to one authority (Moore," Jgs," ICC). In relation to two of the 4th-century recensions its text is neutral, neither predominantly Lucianic nor Hexaplaric; but it has been regarded by some authorities as Hesychian. Possibly the recension made in the country which produced the Septuagint adhered more closely than others to the primitive text; some "Hesychian" features in the B text may prove to be original. Still even its purest portions contain marks of editorial revision and patent corruptions. Codex Alexandrinus presents a quite different type of text, approximating to that of the Massoretic Text. In the books of "Reigns" it is practically a Hexaplaric text without the critical signs, the additional matter being mainly derived from Aquila. Yet that it contains an ancient element is shown by the large support given to its readings by the New Testament and early Christian writers. Individual manuscripts must give place to groups. In order to reconstruct the texts current before Origen's time, it is necessary to isolate the groups containing the three 4th-century recensions, and to eliminate from the recensions thus recovered all Hexaplaric matter and such changes as appear to have been introduced by the authors of those recensions. Other groups brought to light by the larger Cambridge text have also to be taken into account. The attempt to Renetrate into the earlier stages of the history is the hardest task. The Old Latin version is here the surest guide; it has preserved readings which have disappeared from all Greek manuscripts, and affords a criterion as to the relative antiquity of the Greek variants. The evidence of early Christian and Jewish citations is also valuable. Ultimately, after elimination of all readings proved to be "recensional" or late, the decision between outstanding variants must depend on internal evidence. These variants will fall into two classes:
"(1) those merely affecting the Greek text, by far the larger number and presenting less difficulty; (2) those which imply a different Hebrew text. In adjudicating on the latter Lagarde's main axioms have to be borne in mind, that a free translation is to be preferred to a slavishly literal one, and a translation presupposing another Hebrew original to one based on the Massoretic Text."
So if the Codex Alexandrinus Septuagint presents a quite different type of text, approximating to that of the Massoretic Text, then why isn't the Septuagint in Codex Alexandrinus the standard text among Septuagint Onlyists instead of the Septuagint of Codex B??? or is it disregarded in favor of Codex B because it does approximate the Masoretic Text???