21 mm. 3.79 grams.
Salonina, wife of Gallienus who reigned 253-268. Sturck late in the sole reign.

SALONINA AVG
AVG IN PACE, Salonina seated left holing olive branch and long transverse scepter. In ex:  MS

RIC Salonina 58. Normanby Hoard 460, no photo, this coin, from C.J. Martin. Cunetio 1765 (7th series of 8 during the sole reign)

The "M" in the mintmark is one of the very first mint indications, standing for "Mediolanum" (= Milan). When all coins were struck at Rome, there was no need to distinguish mints.
Long ago this type was considered possibly Christian because of the legend ("Augusta in peace"), but now it is not generally regarded as referencing Christianity.

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