Watch List notes:

The Roman Empire

Tacitus, 275 – 276. Aureus, Siscia November 275-June 276, AV 4.01 g. IMP C M CL TACITVS P AVG Laureate, draped and cuirassed bust r. Rev. CONSERVAT – OR AVG Castor standing l. and placing hand on neck of horse standing beside him. C 30. RIC 111 (Ticinum). CBN –, cf. pl. 93, 393. Estiot 26. Calicó 4069 (these dies).
Exceedingly rare, only five specimens known of which only two are in private
hands. Struck on a very broad flan, several light marks and bent
and straightened out, otherwise extremely fine

Ex NAC sale 72, 2013, 735.
The Dioscuri, the heavenly twins Castor and Pollux who were credited with miraculously saving a Roman army at the battle of Lake Regillus in 484 BC, appeared frequently on coins of the Roman Republic, but rarely on those of the empire. In that respect this aureus of Tacitus is intriguing, not only for its artful depiction of Castor, but for its use of that divinity so late in Imperial history. Even more remarkable, though, is the fact that Castor appears on a coin of an emperor who was no juvenile heir to the throne, but an elderly man who had enjoyed a long and distinguished career in the government and the military. If literary references are any measure, interest in the Dioscuri was still reasonably strong in the early years of the Julio-Claudian dynasty, when male heirs often were likened to the Dioscuri. Subjects of that comparison include Gaius Caesar and his brother Lucius Caesar, Tiberius and his brother Nero Claudius Drusus, Tiberius' first two heirs Germanicus and Drusus (who, in fact, was nicknamed Castor), Germanicus' eldest sons Nero Caesar and Drusus Caesar, and the twin sons of Drusus and Livilla. Thereafter, adoration of the Dioscuri faded, with references to them being rare. Except for the purposefully anachronistic 'restoration series' of Trajan, no image of Castor or the Dioscuri appears on coins for the more than two centuries between aurei issued in 41 BC for L. Servius Rufus and sestertii issued for Commodus in AD 177, during his first year as co-emperor with his father, Marcus Aurelius.
Next to portray Castor was Septimius Severus on aurei, denarii, quinarii, sestertii, dupondii and asses that he struck on behalf of his youngest son, Geta, in the period 200-202. For the sake of accuracy, we note rare issues for the Gallic rebels Postumus (aurei and double-denarii showing Castor standing beside a horse) and Victorinus (an aureus using the Dioscuri as the symbol for the tenth legion Gemina).
After this lone aureus of Tacitus, on which Castor is described as the emperor's conservator (protector), the only remaining issues of this kind were struck in the Tetrarchic period. An aureus of Constantius I Chlorus, struck as Caesar at Aquileia, shows the Dioscuri standing, and the rebel Maxentius produced a significant issue at Ostia from 309 to 312. That series, inscribed AETERNITAS AVG N, recalls the era of Rome's foundation by showing the Dioscuri standing beside their horses, sometimes accompanied by a miniature scene of Romulus and Remus being suckled by the she-wolf.