In April 2016 Ebay seller together5882 from Sarajevo, Bosnia & Herzegovina was offering these sample and test designs that were originally handed out by salesmen from various manufacturers trying to get work. He has obviously been able to get all these plates from some police department plate factory in the former Yugoslavia.
Most of them have something wrong with them and all are mint and unused. Most of the designs are wrong, some have numbers that have never and will never exist, some are failed test designs and one is a fantasy design. Some show signs where they have been attached to a frame or wall for display purposes.
Do not buy mint plates or you will end up with rubbish in your collection.