I have looked a bit around, and was wondering if there was any information about which could be the earliest still surviving and accessible (for public) inscriptions which included the Latin initialism SPQR (Senatus Populusque Romanus)? By inscription I mean something carved/engraved on a permanent stationary object, such as on all kinds of architectural objects and elements, versus, eg. writings/engravings on coins, equipment, weapons, armour and other items.
As I understand, inscriptions were commonplace throughout the history of Rome, and a score of them still survive. Wikipedia says that "The title's date of establishment is unknown, but it first appears in inscriptions of the Late Republic, from c. 80 BCE onwards. Previously, the official name of the Roman state, as evidenced on coins, was simply ROMA."
As SPQR has been used since then throughout history and is still used, it is a bit hard to discern which of them are "very old", which "merely old" and which new from the looks of the items inscribed (except for the very obviously modern ones).