Nice coin!
I have one of similar weight, 4.3g, top coin below.
These are actually 2/3 nummus, not 1/2, with the X-VI field marks meaning 16 DC (denarii communes) vs 25 DC nummus. Still, at 4.3g they are almost 50% overweight (closer to 4.57g nummus than 2/3) !
Bottom coin below is a plate coin from Victor Failmezger's "Roman Bronze Coins: From Paganism to Christianity 294-364 A.D", and is an obverse die match to yours.