Thank you for your research yet again. Great, great job.
If you end up looking up retirement letters, I would appreciate a ping.
Once I found fairly clear evidence that the retirement dates were dates certain, I stopped looking. I believe that O'Connor is the first Justice to deviate from the time-honored practice of providing a date-certain for retirement.
Can you imagine doing that at a place of employment? Resign, timing contingent on your replacement being hired? It puts the boss-man in an awkward position.