I have seen a few of these helmets, but this is my first example. The second coin below is a typical helmet.
Constantine I
A.D. 318-9
20mm 3.2g
CONSTA-NTINVS AVG; helmeted and cuirassed bust right.
ROMAE AETERNAE; Roma std. r., shield in lap inscribed X/V, in fields P/R
in ex. RS
RIC VII Rome 146 var.