I take the chronology of LXX from:
CMI : Some remarks preliminary to a biblical chronology
by Pete Williams
This includes the problem, according to his choice of LXX version, that Methuselah "dies after Flood".
Probably this is a faulty reading, but this is the one I access now. And probably it may be not identical to the reading before St Jerome, when he calculated the chronology we read at Christmas vigil in the Catholic Church.
However, it may serve as to indicate who knew whom. First is a chart for pre-Flood overlapping lifespans:
|Adam 0 - 930||=||+||+||+||+||-||-||-||-||-||-|
|Seth 230 - 1142||+||=||+||+||+||+||+||-||-||-||-|
|Enosh 435 - 1340||+||+||=||+||+||+||+||+||-||-||-|
|Cainan 625 - 1535||+||+||+||=||+||+||+||+||+||-||-|
|Mahalaleel 795 - 1690||+||+||+||+||=||+||+||+||+||+||-|
|Jared 960 - 1922||-||+||+||+||+||=||+||+||+||+||-|
|Enoch 1122 - 1487||-||+||+||+||+||+||=||+||+||-||-|
|Methuselah 1287 - 2256||-||-||+||+||+||+||+||=||+||+||+|
|Lamech 1454 - 2207||-||-||-||+||+||+||+||+||=||+||+|
|Noah 1642 - 2592 (after Flood)||-||-||-||-||+||+||-||+||+||=||+|
|S,H,J 2142 - after Flood||-||-||-||-||-||-||-||+||+||+||=|
Adam had opportunity to teach Seth, Enosh, Kenan (the first Cainan being really Kenan), and Mahalaleel about the sixth day of Creation in its details (Genesis 2), about Flood, and about the story of Cain and Abel. During these centuries he had ample opportunity to correct any misunderstandings that might have arisen. He also had ample opportunity to perfect his method of formulating the events in a short and easy to memorise way, supposing, for argument's sake, they did not yet write before the Flood.
Griots of Africa and Serbian Guslars have certainly memorised with perfect accuracy songs far longer than the relevant chapters of Genesis. Therefore, even if writing were a post-Flood invention, we can be sure that the transmission even orally has kept original content intact.
Here will be a chart for post-Flood co-longevity, up to Abraham:
|Noah 600 B.F. - 350 A.F.||=||+||+||(+)||+||+||-||-||-||-||-||-|
|Shem 100 B.F. - 500 A.F.||+||=||+||(+)||+||+||-||-||-||-||-||-|
|Arphaxad 0 - 565||+||+||=||(+)||+||+||+||-||-||-||-||-|
|Cainan 135 - 595||+||+||+||(=)||+||+||+||-||-||-||-||-|
|Shelah 265 - 725||+||+||+||(+)||=||+||+||+||-||-||-||-|
|Eber 395 - 899||-||+||+||(+)||+||=||+||+||+||-||-||-|
|Peleg 529 - 868||-||-||+||(+)||+||+||=||+||+||-||-||-|
|Reu 659 - 998||-||-||-||-||+||+||+||=||+||+||-||-|
|Serug 791 - 1121||-||-||-||-||-||+||+||+||=||+||+||+|
|Nahor 921 - 1129||-||-||-||-||-||-||-||+||+||=||+||+|
|Terah 1000 - 1205||-||-||-||-||-||-||-||-||+||+||=||+|
|Abraham 1070 – 1245||-||-||-||-||-||-||-||-||+||+||+||=|
Obviously, this is NOT the text St Jerome had, since in his text less than 1070 years pass between Flood and birth of Abraham.
This would rather be a text like those behind Byzantine chronologies where Christ is not born 5199 but 5500 after Creation. While speaking of this, I am not sure if St Jerome used LXX all way through or if he looked at his own translation or if he looked at Josephus, but mainly he used LXX.
Post-Flood Cainan (Cain-am, this time, not Kenan) has overlapping lifespans in parentheses, since he is not a trustworthy link of tradition, he was omitted from at least what is now Masoretic text, precisely because he was unjust, he coveted pre-Flood magic lore (I think the source for this is ultimately Book of Jasher, which I have not read myself, and immediately one of those researching in it).
The point remains, sufficient lifespans even just in the direct line (and here omitting uncles and granduncles and such) were there to guarantee a very thorough apprenticeship in Hebrew lore (or what became such) to ensure an even humanly speaking near certainty of good transmission.
Note that 1070 is 128 years more than the Chronology of Roman Martyrology for time between Flood and Birth of Abraham.
Here I have underlined the "time of Peleg", or "days of Peleg" in which it is presumed that Tower of Babel had the ending of its building in a confusion of tongues. It is obviously not there in Roman Martyrology, partly because Bible (in all versions) lacks a precise year, partly because even if there had been such, the Tower of Babel and Confusion of Tongues is hardly a very good epoch to start counting down to the Birth of Christ from. We are, you see, talking of the Roman Martyrology for December 25th, the first feast of which is of course Christmas, celebrating the Birth of Christ in Bethlehem.
Hans Georg Lundahl
(since earlier renamed)
St Charles Borromeo