It's registered as an invalid carriage... meaning a vehicle to be used by a disabled person. There are three wheelers, cars, vans and also there is one Peugeot 406 I am aware of, that uses PZ- plates.
Many of these vehicles were bought by ex-service personnel, who were disabled in action. I believe there were significant tax concessions for these vehicles.
However, it is unclear whether the vehicle will continue to retain PZ- plates or not, when it is transferred, because the purpose of these vehicles is for use by disabled persons and if it is transferred to a non-disabled person, the purpose of the vehicle is changed. Also note that if the PZ- plates are to be changed, the vehicle class also must change (ie. from Invalid Carriage to Motor Car). There may also be taxes involved.
If you are contemplating on buying one, it's best to avoid it.