Let alone that many amateurs use PO Boxes for their legal addresses, which
may be many miles away from their home QTH's (especially if the box is
located in the area they work in, not live in).
Not only can this skew the data that Tim's looking at for his purposes, but
it also explains why many zip codes have changed. Many post offices (I know
the State College PA for one did this) changed the primary ZIP code for
their PO boxes when ZIP+4 was implemented (ie, PO Box XXX 16801 became PO
Box XXX 16804-0XXX).
Other ZIP codes may be very specialized -- certain business that received
huge quantities of mail may have their own ZIP code.
The USPS computer system has some way of routing mail sent using obsolete or
superseeded ZIP codes. Whether or not there is a "master list" made
available to the public, I don't know.
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