My theory on exit holes on bullets. Shoot me down if you disagree.
If a 30 cal/8mm bullet enters a helmet (except for an odd angle glancing shot or high up on the dome) I find it highly doubtful that it would be able to penetrate the helmet and then both sides of a skull and then have the velocity to exit the other side of the shell. That is why I think a helmet like this was not worn at the time. I could be totally wrong so happy to hear from anyone with solid ballistic experience.