At the beginning police plates were similar to normal civil plates of San Marino, with the format RSM 00000 in blu with leading zeros if needed and the print "GENDARMERIA" in red small characters on the bottom (first picture), while front plates had the number in red and no legends (second picture). Later the plate changed the look, with the coat of the arms made with polychromatic techniques on the left, followed by the letters GE in red and then a number of 3 digits with leading zeros if needed in blu, and with the print REPUBBLICA DI SAN MARINO on bottom (third picture). In 1983 the Gendarmeria was incorporated in Polizia, with a similar plate with the full print POLIZIA in place of the letters GE, as you can see in the fourth picture.
In the fourth picture you can also note that, though the plate has the print POLIZIA, the historical name of the Corps, GENDARMERIA, is still present on the back of the car.
The last picture shows a plate of polizia for motorcycle: as you can see the only difference is that this plate is square.
The first three pictures are taken from the archives of ALPCA. Many thanks to Antonio Carretta for the fourth picture and to Paolo Troncatti for the fifth.