Hi Victor,
This coin has been sitting around in my trays for ages without giving it any attention, the fact it is broken probably contributed.
It has all the appearance of an Eastern mint issue but doesn't look like any of the Antioch issues, it also has a very low weight at 1.33g.
Taking into account that approx. 25% is missing would give it a weight somewhere around 1.76g which seems very low to me for a billon Antoninianus, was there such a thing as a Quinnarius issued for him ?