...ok, so i finally got it back together and it's working normally. I wonder if any of you guys know the answer to helix guide tabs in the rear of the lens barrel? One is 'slotted' the other 'plain' ? - I didn't note where they came from but they seem interchangeable. It's also possible to fit them 'inverted' - but then they don't 'block' the helix over the entire focusing range so that's clearly wrong.