Plates for the prison service vehicles started being issued in 1984 and at the beginning they had the format: AdiC 000 AA, where the first letters were red and stood for "Agenti di Custodia" (Guarding Policeman). You can see one of these plates in the first picture.
Picture 1: 1984 - 1992
The second picture shows the plate that was used from 1992 to 2003: "POLIZIA PEN." means "Prison Service" and the format is AA 000. As you can see in the picture, there's a little blue symbol on the top left corner: it's a flame and it's the symbol of Prison Police (thanks to Giancarlo Raposo for this information). There are also some test plates of the prison Service: they are very similar to normal plates, but have the format P·000: unfortunately I haven't got any picture of these plates yet.
Picture 2: 1992 - Today
In 2003 the size of the plates changed and they became much wider (52 cm, picture number 3): for this reason the full "POLIZIA PENITENZIARIA" is printed on the first line. Front plates are smaller than rear (about 36 cm, picture number 4).
Pictures 3 and 4: 2003 - Today (read and front)
The Prison Service has got also plates for motorcycles. They were square with the words "POLIZIA PEN." in two lines and the symbol at the beginning of the second line until 2003 (picture 5). Then, in 2003, the full name "POLIZIA PENITENZIARIA" was inserted, the plates became larger and the symbol was removed (picture 6). The numbering system is different from that used on the cars, with only a serial number.
Picture 5: 1992 - 2003 (motorcycle)
Picture 6: 2003 - Today (motorcycle)
I need to thank very much Jim Fox and Adolfo Arena for the pictures number 1, 2, 5 and 6. The pictures number 3 and 4 are taken from the web site Mezzi di Emergenza.