Over the past weeks I have been reading some books on this very subject. Fascinating.
Indeed, quite fascinating.
The Septuagint can be found in numetous codices, not the least of which are the Codex Vaticanus, Sinaiticus, Aexandrinus and others, all pretty much in agreement with each other, and all much older than the oldest Masoretic text of the many Masoretic texts.