The corps of Milizia della Strada was created in 1928 as a part of the Milizia Volontaria per la Sicurezza Nazionale and after four years, in 1932, the corps changed its name in Milizia Nazionale della Strada. Anyway the letters used on the license plates were always M.d.S.
Plates of autovehicles had the number on the first line and the letters MdS on the second: unfortunately I haven't got any picture of such a plate yet.
Motorcycle: two front and rear
The plates that you can see here in this page were mounted on motorcycles and, as you see in the first two pictures, these had a front plate attached over the mudguard as it happened on the motorcycles of Regio Esercito and of Regia Marina. You can find the full picture I took the detail of the first plate from in the old photo gallery (picture 4).
Ernesto Vitetti sent me a table with all the data of the vehicles of the Milizia della Strada in use on january the 13th 1933: if you want to see it click here. In the table you can notice that motorcycles were many more than cars and for this reason it's easier to find pictures of motorcycles.
Some special plates of the Milizia della Strada were used also suring the military operations in Spain: they looked like the normal plates, but had different colours.
The first picture is taken from the web site "Il museo delle divise fasciste" and I want to thank Luca Maiorano very much for let me use it on my web site. Many thanks also to Edoado Lecis and to Daniele Cacozza for the second and the third pictures, to Guglielmo Evangelista and to Paolo Mazzini for their history and explanation about this subject.