Polybios about thorakitai in Seleukid army during the eastern anabasis of Antiochos III (Book X, 29, 5-6):
„Having made this plan he gave the command of the first division to Diogenes, entrusting him with archers and slingers and those of the mountaineers who were expert in throwing javelins and stones, who also, whenever time and place called for it, fought singly and rendered most useful service on difficult ground. After these he placed about two thousand Cretans armed with bucklers under the command of Polyxenidas of Rhodes, and
lastly the light troops armed with breastplate and shield under Nicomedes of Cos and Nicolaus the Aetolian.“
So the thorakitai are described as light troops (most probably as compared to heavy Macedonian phalanx) with breastplate (thorax) and shields (most probably thureos shields) used several years before Antiochosī Roman war.
Polybios about infantry armed in Roman fashion in Seleukid army during the reign of Antiochos IV at Daphne parade (Book XXX, 25, 1-3):
„This same king when he heard of the games celebrated in Macedonia by Aemilius Paullus the Roman general, ambitious of surpassing Paullus in magnificence sent out embassies and sacred missions to the towns to announce the games he was about to give at Daphne, so that people in Greece were very eager to visit Antioch then. The festival opened with a procession composed as follows
: It was headed by five thousand men in the prime of life armed after the Roman fashion and wearing breastplates of chain-armour.“
B. Bar-Kochva discuss this passage in The Seleucid Army on page 60.
1 Maccabe about the Seleukid army at the battle of Beth Zechariah during the reign of Antiochos V and regency of Lysias (6, 33 - 39):
„ Early in the morning the king rose and took his army by a forced march along the road to Beth-zechariah, and his troops made ready for battle and sounded their trumpets. They showed the elephants the juice of grapes and mulberries, to arouse them for battle
. And they distributed the beasts among the phalanxes; with each elephant they stationed a thousand men armed with coats of mail, and with brass helmets on their heads; and five hundred picked horsemen were assigned to each beast. These took their position beforehand wherever the beast was; wherever it went they went with it, and they never left it. And upon the elephants were wooden towers, strong and covered; they were fastened upon each beast by special harness, and upon each were four armed men who fought from there, and also its Indian driver. The rest of the horsemen were stationed on either side, on the two flanks of the army, to harass the enemy while being themselves protected by the phalanxes. When the sun shone upon the shields of gold and brass, the hills were ablaze with them and gleamed like flaming torches. “
B. Bar-Kochva discuss this passage in The Seleucid Army on pages 180 – 183, in Judas Maccabaeus on pages 128, 313 - 316.
And also after a little search I found the picture (see the attachment) of Hellenistic terracotta found in ancient city Nippur, now in Pennsylvania museum (during Seleukid period the ancient city was turned into a fortress), which is located in southern Babylonia (Antiochos IV with his army is attested in this region during his eastern anabasis). The terracotta most probably is model of infantryman from Hellenistic period with thureos shield with sword in the right side as Roman soldiers were using (the Hellenistic soldiers are shown to were their swords on left side). In Republic era the Roman army didnīt campaign in this area. The first Roman army which campaigned there was under the emperor Trajanus during early 2-nd century AD. As the thorakitai were a Hellenistic development based on confrontation with Celtic/Galatian warriors, so I think posted written ancient sources and this terracotta may be describing the Seleukid infantry unit armed in Roman fashion.