KINGS OF THRACE. Rhoemetalkes III, circa AD 38-46. Diassarion (Bronze, 23 mm, 8.06 g, 7 h), with Gaius (Caligula), 37-41. [ΓAIΩ KAIΣAPI] ΣEBAΣ[TΩ] Laureate head of Gaius (Caligula) to left. Rev. BAΣIΛEYΣ POIMHTAΛKAΣ (retrograde) Laureate head of Rhoimetalkes III to right. Leu Web Auction 1 (2017), 690. RPC I -, cf. 1724 (larger denomination and reverse legend not retrograde). Very rare. Good fine.
This is apparently the half denomination to RPC I 1723 and 1724, but what makes this coin particularly curious is the retrograde reverse legend. The die cutter was also likely responsible for the reverse die of RPC I 1725 with its retrograde BAΣIΛEΩΣ legend.