The letters were "postmarked" in New Jersey and spores were found in and around the machinery involved. No spores were found anywhere else in New Jersey. The only place we have any mail acceptance activity showing contamination is Florida.
Single piece rate First Class Mail could be mailed anywhere and still be "postmarked" in a different place. This is not unusual!
Yours is an interesting theory which could be possible for the first 2 letters postmarked in September. Weren't the Daschle and Leahy letters mailed and postmarked a few weeks later in October?