Poor guy

Six pounds. That's gonna leave a mark.

It may be for something inside, maybe a mounting point for straps or padding of some sort.

Could be . Maybe a clip-on for a shield mount that must not have been there. But it's a good idea.

I looked into it, it is for straps that go over the shoulders, there was one on the other side too but you can see it smashed flat.
Here is a picture with the straps.
http://bottega.avalonceltic.com/catalogo/prodotto_1099/armature_elmi_scudi_parti_di_armatura_busto_da_corazziere_austriaco.php

That's awesome! Thanks for the link.