In the Prima players guide it says this mask is +50 Heavy Armour, but it turns out to be resist poison and disease. I take it that there is no Priest mask with +50 armour

You can smith/improve them. But I don't recall seeing one that added +50 to armor. But I don't use the masks even though I have them all.

isnt that the gold mask? I think it also has a small chance to summon a dragon priest while wearing it.

no, that's a different mask .