Centurion helmet from Sisak, Croatia

[Vindex]

The issue with the neckguard for instance, the Gallic C has a larger sloping neckguard. It appears that in the supposed Gallic series, it is the D and G with the smaller neckguards and the C and H with the larger neckguards. I dare say more so than the Sisak helmet you are showing. It appears that the Sisak helmet's neckguard is in between the known Gallic F and the Gallic C,D.

There's also the Brigetio variant of course which is classed as "J" and is in the museum at Caerleon. (Apologies for the poor photograph but the lighting in the museum is not good!)