267-268 AD., Gallienus, Rome mint, emission 6, officina 9, Æ Antoninianus, Göbl 743b.

Gallienus, Rome mint, emission 6 (10?), officina 9, 267-268 AD.,
Æ Antoninianus (ø 16,5-17,5 mm / 2,36 g), bronze, axis coin alignment ↑↓ (ca. 180°), small flan,
Obv.: [G]AL[LIE]NVS [AVG] , his radiate head right.
Rev.: [NEPTVNO C]ON[S AVG] / N , (“Neptune god of the seas, preserver Augusti“), hippocamp swimming right, N in exergue (looking like III , at that era at the mint of Rome the letters M, N and V disintegrate into single I-fragments).
RIC V(1), p. 152, 245K (sole reign) (common) ; Göbl MIR 743b ; Cunetio 1392 ; Chalfont hoard 467 (11 pieces) .

Part of the so called “Gallienus zoo series“.

Issued in 267-268 AD. to commemorate vows to Neptune invoking his protection against the revolt of Aureolus. The hippocamp is a mythical beast consisting of the foreparts of a horse and the sea-serpent tail. They were the chariot-beasts of Neptune.
In later times the mint of Rome used a letter abbreviation for the Latin number of the officina, such as P, S, T, or Q (prima, secunda, tertia, quarta). During this early period though, it was a more mixed system, using a combination of a Greek numbering scheme and Roman. Officinae numbers 1-8 used Greek numerals, while 9 used Nu = N, which normally meant 50. (The normal Greek letter for 9 was Theta = Θ). Officinae 10-12 went back to typical Roman numerals, providing a mixed and sometimes confusing pattern. Soon after the reign of Gallienus the Imperial mints seem to have ironed out their system more, with western mints using the Latin numerals while the eastern ones used Greek, but the Gallienic period provides an interesting glimpse into the development of this system.