If you can find an older copy than the Septuagint translation feel free to let me know.
History is history.
I just told you that the text of each and every kosher Torah Scroll, which is unpointed and unpunctuated, goes all the way back to Sinai. Naturally it is older than any pointed text. And that's what the Massoretic text is--a pointed text. The Massoretes never wrote or rewrote the Hebrew Bible.
History is history.
Unfortunately for you.
You might want to check out the Samaritan Pentateuch. This seems to date from about the 2nd century AD, and so antedates the canonical Masoretic text by more than half a millennium. In addition, where LXX and the MT disagree, the Samaritan version in almost all cases accords with LXX.
This is pretty good proof that (a) the Septuagint is an accurate version of an older tradition, which the MT changed, and (b) the changes in the MT are certainly post christian. All of which is further historical confirmation of your claim.