RE: Let me take a stab at it first. Mark 16: 9-20, were not in the two oldest manuscripts.
OK, so what did we have in the oldest manuscripts and which Chapter and verse did the manuscripts end?
Oldest was not necessarily best, as many wrongly presume. Oldest may mean the text was so unreliable that it was just left on the shelf, never got used amd worn out/recopied, and so survived simply because it was such a poor reference. And since the passage was one selected by a great plurality of the churches across the Christian population, one may assume rightly that the oldest was the worst according to a plurality of scholarly readers, not the best.