Time to ruin definitively my surviving reputation (if I ever had one!) on RAT Forums!
Two premises, the first: thanks Nathan for the pic of the bas-relief of Severius Acceptus of the Legio VIII Augusta, III century AD:
The second, is a relief from Campania belonging to the I century BC.:
...So here the infamous work I've done, a legionary of the III century wearing not a Morrione of the XVI/XVII century of course, but nothing less than the mysterious Attic Helmet!!!...
Yeah! This is my personal interpretation of the relief of Severius Acceptus, his helmet is a derivation from the Gladiatorial Thrax helmet.
This solution offers us two advantages: We know that they were able to produce this helmet, we have the models perfectly preserved of the Gladiatorial helmets.
So we have only to find the derivative military model, but we know exactly where it came from!
IMO the relief of Severius is one of the first images of the Attic helmet, so we can assume it started to be used during the III century, and we should remember that Marcus Aurelius, during the first Marcomannic war, enlisted also slaves to defend the Limes of the Empire afflicted by the great plague.....
Side note from the architect: If you project the side view of the Thrax helmet on a surface parallel to the longitudinal axis of the helmet, as to say if you falattened on a surface the lateral view of the Thrax helmet you get exactly the bas-relief of Severius!
mile: