I have mine in front of me (took the day off).
Mine screwed back into position correctly.
With the front section out:
1) Make sure thre threads are clean of dirt, oil, paint, etc.
2)Make sure the aperture ring is turned to open the blades fully: that is F2.
3) The front section should screw back into original position, the index will line up with the F2 mark. This takes a lot of tightening, I used a rubber sheet.
It is possible that the Index marked SLIPPED when you took the lens off. It is held in by the three set screws in that ring.
4) Tighten the lens as far as possible. Again- took a lot on mine to line it up.
5) TEST the lens with film, or at the film gate of the camera with a test strip- scotch tape on exposed negative is what i use-.
6) If the focus is good at Infinity and Close-up, back off the tiny set screws and move the index mark so it lines up with the F-Stop ring.