The genealogy ofin Genesis 5 as it appears in the Septuagint is very different from the geneologies of the Masoretic & Samaritan texts. For example, Methuselah lives 14 years past the flood in the Septuagint's genealogy, but he dies before the flood in the other geneologies. The Septuagint also adds over a thousand extra years to this geneology. The following article lists the differences between the texts.
My question is where the Septuagint got those numbers from.
My conclusion is that the Septuagint was reconstructed from older manuscripts, which means that those manuscripts should contain the vastly different genealogy. I'm guessing that the answer to this question can be found in one of the critical editions of the Septuagint, but I am not in possession of this particular book.