(19.5mm, 2.42 g, 12h). Rome mint. 8th emission, circa AD 264-265.  Good VF. The second known example.
The reverse type of this coin has been studied by Jean-Marc Doyen (Recherches sur la chronologie et la politique monétaire des empereurs Valérien et Gallien, vol. 2a: Etude des émissions monétaires de Milan, PhD thesis, Louvain-la-Neuve 1989, pp. 101–3, available online), and it is copied from a rare series of Antoninus Pius, struck in AD 140 (as ref. RIC III 694a and aureus ref. Calicó 1689) which celebrated with some advance the 900th birthday of Rome. It was first used by Gallienus in 260, with the legend TRIB POT VIII COS III and a laureate bust left, on a coin struck in Mediolanum (Milan) (ref. MIR 36, 945gg = Doyen 49), and again with the legend TRIB POT COS IIII (MIR 36, 946gg = Doyen 71). It shows Mars descending toward Rhea Silvia, daughter of King Numitor and a Vestal virgin who is depicted as sleeping in the forest, just before raping her. This mythological episode (told by Ovid, Fast. III.V.11 and Livy, Ab Urbe Condita I) is of the utmost importance for the story of Rome, as the intercourse led to the conception of the twins Romulus and Remus, legendary founders of Rome. The same iconography can be found on wall paintings from Pompeii and in the baths of Titus (see LIMC II/1 pp. 459-51 and II/2 pls 415-6), but also on mosaics, reliefs, silverware, and gems.