It can be hard to differentiate a D2 "visor" (fixed browband) from the riveted edge of a D6/D7 ridge helmet on a worn coin like this, but ignoring that there are three things pointing to D7 rather than the Attic/Praetorian D2 helmet.
1) It seems to have the star decoration(s) characteristic to the ridge helmets.
2) It has the tall feather crest vs the normal horsehair crest of the D2 helmet, although at the beginning of this Rome P-R/vota series there are rare D2 helmets for Licinius with a feather crest.
3) In terms of type/helmet sequencing in this series, this 5-line vota type is early, but the D2 - for Constantine - is late. Early D2 is basically for Licinius, although there's an exceptional Pax Perpetva for Constantine with D2 helmet that could be interpreted multiple ways.