No person knows what the Septuagint is because there is no such thing as a pure, unadulterated, Septuagint.
What Greek version of the Old Testament did the Apostles use?
The Septuagint was translated by Greek-speaking Jews, supposedly rabbis, and supposedly under inspiration, just like the Hebrew scriptures. Again, it was good enough for the Apostles.
The key word is "supposedly". Legend is legend. Fact is fact. "Fact" is absent.
I agree. There is no such thing as pure unadulterated scripture, period.
What Greek version of the Old Testament did the Apostles use?
The one found in the New Testament quotes, which agree with the extant version of the Septuagint.
The key word is "supposedly". Legend is legend. Fact is fact. "Fact" is absent
Sure, but that is true of all scripture, which is why they must be believed. Obviously a Greek-language set of scriptures existed since the New Testament writers quote from them. It just so happens, as a matter of fact, that the same verses found in the Greek Old Testament found in various 4th century Codices correspond to them.
I suppose you could say that Christians wrote the Septuagint in order to make it "fit" the references made by New Testament writers, but that still doens't explain where did the same writers quote from; certainly not from the Hebrew Bible. So, clearly another set of scriptures existed, which Josephus and Philo refer to as the Septuagint.
All of them are 1st century sources. Ergo, there was such a thing as the Septuagint in the 1st century AD, despite the groundless claims of some to the contrary.