There was no such thing as an Ancient Roman Flag as we know it nowadays. What the Romans used instead of a flag were battle standards of various sorts and forms. Many public buildings had inscriptions on their facades. The letters S.P.Q.R. Senatus PopulusQue Romanus stood to tell everyone that what they had before them belonged to the Senate and the People of Rome. https://mariamilani.com/ancient_rome/ancient_roman_flag.htm