It's weird that the plate is white, but it seems it isn't legit/original, it looks like paper...
Nonetheless, the vehicle is actually registered with that letter combination, according to the online public accessible insurance registry. According to it, the plate ZN9254 used to be registered as a trailer, which would explain why it's white, but I don't know how they managed to legally trasnfer the plate to a bus, and how to get a second plate to complete a pair since trailers only get one plate.