According to the background, these are fixed and do not move with the wearers head. However, that is certainly not the case if you look at the art, and there is an old metal figure of an MK3 marine looking left.
So whats the consensus? Fixed or moves with the marine's head?
All the armour varies really. Across legions and individual suits.
As I see it they developed defined 'marks' and distributed them. You can expect initial deployment to be whole, but the era we are dealing with (TGC > HH) had up to 6 marks in circulation and very much mixed, matched, patched, improvised and artificed into all sorts of hybrids and legion-tailored decoration.
Anyway, Mark II did have some fixed helms, but as per the quote:
"The early Mk II helmet was fixed in place like those of primitive diving suits."
...Suggests the 'later' Mk II helms were not fixed. This, to me, says the other metal models are simply later Mark II variants.
I would think that a fixed helmet design on a suit of combat armour wouldn't have lasted long - what's the point in making soldiers with super reflexes then sticking them in a walking coffin. :P