Can´t see in the shots why the helmets should be impossible to remove, it looks like a baniset-style helmet with a pointed visor, like this one:
Baniset with Visor
Of course, I´ve only seen it in the screenshots you put up, and if you don´t have the model anymore, well, that´s bad luck for me (and others who happened to like it), I suppose.
And as for the time period - sallets like the one used as reference for the later version came not into use until the Late Middle ages and mainly in Germany as part of the Gothic armour style, see here:
Gothic Armour
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