The first trial he was convicted on was based on eye-witness evidence, and overturned because of one mystery eye-witness that came up after the fact out of nowhere. The new evidence is period documentation. That is pretty damning and hard to fight..
What do you mean by period documentation? It all still comes down to an eyewitness seeing him there and after 60 (or even 40) years, that’s going to be tough.
As I recall, a ID card came to light showing that he was never at the Camp where “Ivan” engaged in those atrocities.