The first military vehicles had normal civil plates, with a mark "Servizio militare" (Military Service) since 1910 for exemption of taxes.
The first real military plates were introduced in 1914: they were rectangular, with white background, the letters S.M., SM or Szio Mre in red (seldom in black... see the first two pictures: you can also see the full picture the second plate is taken from in the old photo gallery, picture 13) and a serial number in black, they usually were hand painted and attached also to the side of the vehicles.
Autovehicles: two different plates
The motorcycles of Servizio Militare at the beginning had a square plate with SM on the first line and the number on the second, and the numbering system was the same used for the other vehicles. After some years these plates disappeared and the motorcycles didn't have any plate, as some pictures of the time show.
During the First World War another red letter was added after the number:
Plates of Servizio Militare were used until 1930 (?) also for the vehicles of the colonial forces in Africa. In this case the letters SOM SM (Somalia) or T SM (Tripolitania) in red were followed by a number of three or four digits in black (third picture). The plate could be made of one or two lines (in this case the number was on top) and in the last years plates with swapped colours, black background and white digits, were also used to look like the new Italian plates introduced in 1927 (fourth picture).
Plates used in Tripolitania
Plates of Servizio Militare were abolished in 1927 and substituted by plates of Regio Esercito.
I want to thank very much Roberto Pola for the second and the last picture, taken from the book "La motorizzazione dell'Esercito fino al 1943" written by Ceva and Curami, and Guglielmo Evangelista for the first, taken from the Ufficio Storico della Marina Militare, for the third and for the fourth picture, and for his history of SM plates.