Among the defensive weapons of the Roman army, the breast plate mentioned by Polybius as the most widespread protection among the legionaries of the middle republican period, is certainly among the most elusive for a reliable reconstruction. In fact, there are no Roman finds referable to this protection, while the Italic specimens are much older (VIII-VI century BC) or differ in form and characteristics from the definition of Polybius. In fact, he defines it as a plaque, square, about 22.5 cm side, placed to protect the chest and worn thanks to corrects in leather, so something different from the circular plates, however older, with short anatomical armor sannite and magnogreche, or from the so-called trilobate. And, of course, it is not even the pectoral plates of Villanovan tradition, which, although similar in size and shape, date back to the IX – VII century BC, about 500 years earlier than the legionaries described by Polibio.
An indirect testimony of the use and perhaps even partially of the appearance of this pectoral projection could perhaps be derived from this famous relief on the theme of the Augustan age, from the Tiber.
In this period the gladiators used still predominantly, if not exclusively, a similar armament or almost identical to that of the legionary contemporaries. Without going into detail, this is confirmed by the representation of the Weisenau helmets, the convex shields with a wooden spine, the archaic or Hellenistic type of greaves, typically military and different from those that will then develop exclusively for gladiatorial use.
It is therefore possible that the pectoral protection worn by the two provocatores, clearly inspired by the aegis (egida) of Minerva, complete with Gorgone, is also representative of a element present in the legionnaire kit, if not contemporary, at least of the past then just passed.
Unfortunately at the moment it could be a suggestion without the possibility of confirmation.
Deepening: Gladiators and Caesars. The Power of Spectacle in Ancient Rome, Eckart Köhne, Cornelia Ewigleben, Ralph Jackson. University of California Press, 2000