Antoninus Pius as Caesar under Hadrian
February - September 138
IMP T AEL CAES ANTONINVS, bare head left
TRIB POT COS, Diana standing right, holding arrow and short bow
138 AD must have been a strange year. Rome was ruled by Hadrian, at this point sickly, childless, and recently widowed. To avoid a succession crisis, Hadrian had adopted Lucius Aelius who, incidentally, turned out to be in worse health than his emperor, and was dead by the beginning of 138. Hadrian then turned to Titus Aurelius Antoninus, a trusted colleague and first cousin once removed to his dead wife Sabina. Hadrian then died seven months later at the ripe old age of 62, and Antoninus was made emperor. The Senate initially wanted to decree a damnatio against Hadrian (he was more than a little unpopular over the rash of executions he had ordered,) but Antoninus pleaded tirelessly to have his adoptive father consecrated, threatening to abdicate if they refused. Moved by his show of filing piety, Hadrian was deified and Antoninus was given the honorific name "Pius", "The Faithful".
Surprisingly, despite their relative scarcity, these issues are not in great demand, and thus relatively inexpensive. Additionally, there is a slightly more rare coinage issue of Antoninus as Augustus before he was granted the honorific Pius.