The letters I, O, U, Q and Z were never used in either prefix and suffix number plates. Each had a very reasonable explanation though. 'I' and 'Z' letters were reserved for Northern Ireland issues and the 'O' wasn't (and still isn't) distinguishable from a zero. It was also deemed that the 'U' was too similar to the 'V' to be used. 'Q' reg vehicles were used in special circumstances where manufacture year of the vehicle was not known.