The plates in the pictures are, in the order, a rear and a front plate used in 1960 during the Olympic Games in Rome and they are made of some type of thin cardboard. In the middle of both them you can see the five Olympic rings and a wolf feeding Romolo and Remo: this is the symbol of Rome. According to the legend the wolf saved the two babies and fed them with her own milk; later they fought and Romolo won and founded Rome. In the blue stripe between the rings and the wolf (visible only in the second picture) you can read "MCMLX" (it's 1960 in romanic numbers).
This kind of plates does not appear anywhere in the official documents: they were used only in a restricted part of Rome and with also a normal Italian plate, as you can see in the old photo gallery (picture 35).
I really need to thank Jim Fox, Peter Auckel and Roberto Pola for these good pictures.