The three pictures that you can see here show three various plates from San Marino. Unfortunately I don't know anything about them yet, but the meaning of them: MA stands for "Macchina Agricola" (agricultural vehicle), MO for "Macchina Operatrice" (work vehicle) and it's quite obvious that the last one was used on a tractor (trattore). I'll try to find out more about these plates as soon as possible.
I want to thank Marco for the first picture. The other two pictures are taken from the ALPCA archives. If you have got better pictures or information about these plates, please write me.