This is a follis - at this point in time a small bronze coin with thin silvering - stuck in Thessalonica in Greece in AD 324. 18mm and 3.3g.
Obverse: CONSTANTINVS IVN NOB C, Laureate, draped and cuirassed bust left
Reverse: VOT/·/X in three lines within wreath, TSBVI
I have it noted down as RIC VII 128 - which I suppose is true enough, but RIC (Roman Imperial Coinage) is a reference which few people have actual access to, so many dealers and collectors just accept whichever number is noted down at face value and don't double check :)
VOT X on the reverse refers to a vow to rule for 10 years. In reality, Constantine II was only ruler for 3 years - 337 to 340. This coin was struck when he was 8 years old.
