The corps of Milizia Portuaria was created between 1923 and 1924 and it had the role of sea police, guarding the main ports, in both Italy and the Italian colonies in Africa. The corps was dissolved after the second world war, like all the other fascist militias.
The picture, though very small, shows a plate for motorcycle of the Milizia Portuaria, with the letters M.P. followed by a number. In other cases maybe the letters M.Po. were used, but I haven't got any documentation about this yet.
If you want to see the full picture I took the detail of the picture from, just click here.
I want to thank Roberto Pola so much for sending thsi picture to me.