I had a similar issue to you and having the exact version of Unity they state fixed my issue. I was able to get it from here, you just have to scroll down a little. https://unity3d.com/get-unity/download/archive The build.py script seems to look only for that specific version so having anything else doesn't work. Once I ran buil.py with the correct version of Unity, I was able to import the plugins into a newer version of Unity without any errors. Just a note, I don't believe the file size discrepancy you mentioned is an issue as before and after I had the correct version of Unity my core file was 151kb.