All the letters were mailed in the same place by the same folks. That place is Boca Raton. In fact, with respect to one of the letters the complete track from the point of entry to delivery was found, including a "backtrack" in the route necessitated by way of the fact National Enquirer had changed their address.
There was anthrax the whole way. However, the envelope's had just then been filled, so they weren't leaking as bad as they would be a month later when they hit New Jersey and DC.
Your concern with where the letters were postmarked (which applies ONLY to those letters which were recovered) means nothing when it comes to single-piece rate First-Class letter Mail being sent by an individual using a handwritten address to another individual who is not a utility nor a creditor.
These letters were exceedingly rare types in the modern postal system.
The USPS handles such mail poorly. In fact, if these pieces were mailed anytime from 6 PM Friday, September 7 to 9 PM, Sunday, September 9, they would be just about the only mailpiedes in the collection boxes were they were dropped. If we but assume for a moment that the folks who carried out the attack sought to "hide" what these pieces were by dropping them in individual street collection boxes, the letters would, in fact, constitute the ONLY mailpieces in each collection box. That is, they'd be nearly alone, and readily missed due to the way the MVS or collection route drivers pick up mail during that Friday thru Sunday "dead zone".
The driver stops, opens the box, pulls out the flat tray into which all the mail has dropped, puts in another, empty flat tray, and returns to his vehicle placing the flat tray from the box in the back of his vehicle. At the end of his route he will have several such trays stacked up.
If he didn't see any mail (handling this in the dark can be a problem you know), and a single piece being readily overlooked, these trays will be stacked together and thought of as EMPTY.
When the collection vehicle returns to the post office the trays will be dropped at the backdock for later disposition.
If it looks like there's a stack of empty trays neatly nestled into each other, there's a good chance they will be categorized as empty equipment and end up stacked with others, wrapped in plastic, and dispatched on an empty star route (Highway Contract Route) for transportation North to someplace that generates mail. In this case, they'd go to Philadelphia from West Palm Beach, and from there they'd be sent to major commercial mailers in Eastern Pennsylvania and Central New Jersey.
Once the pieces were discovered in the trays at the mailer's letter shop, they'd be set aside for separate entry in a letter tray. This happens every day at every major mailer's operations ~ there's always live mail (a piece here and there) found in empty equipment. Mailers complain about it constantly.
In New Jersey there was no contamination of the equipment used to separate collection mail by size, so that meant this stuff entered into the 010 operation in a tray all prepared for cancelation.
Now, remember those trays that these letters sat in during their long trip by truck from Florida? They contaminated the trays which, after being used to transport the commercial mailer's mailings to the post office for further distribution, were then sent into other operations for further use. At least one of them ended up in a collection box near a New Jersey university. The contamination was detected. Another was sent over to a carrier route at Trenton, possibly a direct from the commercial mailer to that route. She got contact anthrax from handling the contaminated tray.
BTW, my "belief" is based on 38 years of virtually full time examination and evaluation of postal operations and the preparation of regulations for use by postal employees and the general public for proper use of the postal system in this country. If it's "my belief" it's a pretty good one, and much more educated than that of a mere layman such as yourself.
Inasmuch as my thoughts on the matter are far from baseless, may I encourage you to pay attention to them since they explain every particular from the attack in Florida to the attack in New York, to the attack in DC.
BTW, when USPS left contaminated equipment in the system, a letter or two got contaminated and ended up infecting a lady in Connecticut who died.
I think your ideas about the trays and cross-contamination are very interesting. However I'm not quite following you about the postmarking. Are you saying that mail with pre-printed stamps are handled differently, I presume, than other letters would? If one mailed a letter from Florida with a pre-printed stamp to New York or D.C. addresses it would not be post-marked in Florida but in Trenton NJ?