I would guess that the system generates more digits - lets say 3 when the postal system checks that PZC on the mail, if there is nothing written by the PZC the

I would guess that the system generates more digits - lets say 3 when the postal system checks that PZC on the mail, if there is nothing written by the PZC then it's assumed to be fresh, and the 3 extra digits are looked up via a postal-service-only gateway, then placed on the mail by the PZC for future lookups, possibly along with a date. If the PZC has the extra digits, then it can be looked up and confirmed as redeemed with those extra digits, and possibly the stamped date. The extra digits could be stamped or handwritten, possibly with the postmark date used as the redemption date manually entered from the mail as it passes through the sorting system the first time, OR stamped from the date it's registered as redemption, either would work. Such a system would relatively guarantee that fraud would be difficult, since only one person could have a PZC without the check digits go into the system, and sharing would be useless.