I received my J1 waiver notice recently. The received date is Oct 2003, the time I submitted waiver. Now on the duplicate notice, received date still Oct 2003, while notice date is Sep 2007. The immigration officer may grant my 485 based on the fact that I submitted waiver in 2003.
On a separate assumption, when a 485 receipt is lost in mail and the petitioner requests another copy, he will receive a duplicate 485 receipt with the most updated Notice date but same received date. So AC21 180 days can not be judged on the most recent notice date.
Just two cents.