MP, I would have figured the shield would have been adopted sometime after Antiochos III, at least that is my impression since Livy describes them as similar to the cataphracts at Magnesia. About the closest image I've seen to a Seleukid hetairos would be the cavalryman on the back of a few of Seleukos II's coins said to represent Seleukos himself (the diadem is the give away). No shield, but on one of them appears to be those nifty segmented boots we see on a few statuettes.