A very recent e-mail from my daughter says that one of the mail carriers said (I'll have to e-mail her for her source) that the mailbox in question has been used by mail carriers to "park" other pick-up boxes (those white plastic bins) in the mailbox in question while the mail carriers do their routes. There could've been cross-contamination, but as far as I know, there are still over 30 mailboxes left to test, and, as far as we know, this is still the only mailbox to test positive for anthrax.
I don't know if they mean the mailboxes left to test are in the Princeton area, or those are mailboxes that feed into the Hamilton Township sorting facility, a much wider area than Princeton Borough and Township.
Thanks for the great inside info.