Provisional plates are made of paper and in the past they were often used because a lot of time was required to get normal plates: today plates are given much more quickly and so provisional plates are mostly used for historical vehicles that are not registered and need a plate for exhibition.
In some places it's written that provisional plates for motorbikes were introduced in 1959: anyway the first picture shows a provisional plate issued before that year and it's very similar to provisional plates for cars issued between 1933 and 1959. In the second picture you can see a provisional plate issued from 1959 up to today. The format is the same of the plates for cars, with a number of 5 digits with leading zeros if needed on the top line and a code A0 on the second: the letter says the region and the number the province, as you can see in the table.
Picture 1: 1933-1959 (?)
Picture 2: 1959-Today
The little prints on the right bottom corner, made with pen, are not part of the plate, but were added by the owner of the plate when he used it as a note-pad!!
Many thanks to Alessandro Libanore and to Giancarlo Raposo for these two pictures.