1. yes, mine is the same; paid stamp with date on it.
2. if your number is less than 10750 which is announced, you can even send the package in October, based on 50-day rule in advance (just my opinion). you do not need to wait for 2nd letter, you can do your medicals now.
I am unaware of the 50-day rule. If you don't mind, can you please elaborate.
Also what info does the 2nd NL contain? Could I just get my medicals done and send the AOS package (with all documents and forms Izzy120 listed on post #626) to the Chicago lockbox?