Interesting article about the Kalkriese mask!

[LUCIUS ALFENUS AVITIANUS]

I think if you put a leather thong or a cord around your head the subjection of the mask is better than if you only subject it with the cheekpieces. If the mask moves a little up or down, you even can get out of vision, because you can loose the eye alineation with the eye holes in the mask.

So, i think the lateral holes are made to put a "auxiliar" leather or cord to tie back in the neck and help to a tight position of the mask.

In our group we have the deepeka helmet with mask, and frequently the wearer looses the alineation with the eye holes, and get blind.