The correct answer to this open question is the following.
These letters were the universal mark of Roman authority: SPQR.
These Roman letters were an acronym. They meant "Senatus Populusque Romanus." The translation into English would be "The Senate and the People of Rome." And usually, it had the symbol of the Eagle, the official emblem of ancient Rome. Historians considered that the first time it was used was around 80 BCE.
