I think it's at this point that the FBI STOPPED consulting with postal officials and experts.
Here's the situation ~ they found anthrax contamination in and on the collection box.
There are two ways this can happen. One is that a letter or two gets mailed and releases contamination on the way in. The other way is that a contaminated mail collection container is placed in the box. It releases anthrax spores that then contaminate the inside of the box and the letter drop area, and some may even waft outside and contaminate the outside.
We do not know that anyone ever mailed an anthrax letter at that collection box but we know with certainty that hundreds, if not thousands or even tens of thousands of contaminated postal letter and flat mail containers were released into the postal system from Brentwood facility LONG BEFORE the first case of anthrax was discovered.
We also know that the closest MPC (Mail Processing Center) to Princeton is in Central New Jersey and it was thoroughly contaminated with anthrax that then contaminated every letter and flat tray coming into or leaving the building. That particular MPC served Princeton (among other places).
The FBI simply discarded the idea that a collection box could have been contaminated by a piece of contaminated postal mail handling equipment, to wit, a letter or flat tray.
New flat trays are placed inside each collection box every day!
If this is the sort of evidence that the FBI hoped to use against Dr. Ivins they'd had best hope that no one who'd ever worked for USPS sat on a jury.
Do different post offices use different stamps? This determination sounds a little farfetched to me, but maybe it's possible. If so, I'm surprised that this information never came out until now.