Crispus, Caesar AD 317-326 (son of Constantine the Great)

• Obverse: FL IVL CRISPVS NOB CAES, laureate, draped and cuirassed bust left

• Reverse: PROVIDEN-TIAE CAESS, campgate with two turrets, star above, rows of stone blocks, dot in doorway

• Exergue: RS — Rome mint, 2nd officina (S)

References:

• RIC VII Rome 266, rated R1 (rare) — "Crispus AE follis. Rome. 324-325 AD. FL IVL CRISPVS NOB CAES, laureate, draped, cuirassed bust left... PROVIDEN-TIAE CAESS, campgate, 11 rows, two turrets, star above... Mintmark RS. RIC VII Rome 266, rated R1" 

• Same type described as "Obverse inscription: FL IVL CRISPVS NOB CAES" with "Reverse inscription: PROVIDEN-TIAE CAESS" 

• Wildwinds notes the bust left / RS combination specifically