You will see these 4 letters literally everywhere you look in Rome.
Its Latin and is an abbreviation for "Senātus Populusque Rōmānus" which in English translate to "The Roman Senate and People". Basically its a stamp to indicate something was built or sponsored by the government at the time.
I love the fact that modern Rome as adopted it for exactly the same purpose, so you see it on modern man hole covers etc. There is one right outside the door of our apartment.