Donate to the Lead Adventure Forum to keep it alive!
Think it just depicts a detail shown on the Bayeux Tapestry, generally assumed to be some kind of chest flap / panel to cover up the opening of the hauberk or byrnie.
Nobody knows (like many things in history and archaeology) what the square panel on the hauberk is - the two main contender theories are:1) a square of extra chain and padding to double the thickness of the chest armour in the area where there may have been a slit to get the head through (and the chest is an area vulnerable to stabbing rather than being slashed). Most contemporary pictures show the square patch worn on the chest and surrounded by a leather(?) strip with round fasteners(?). Some eastern armour would feature an extra layer over a mail coat in the form of either a lamellar or scale 'vest' to show that the chest was indeed a vulnerable spot that need extra protection2) a flap to pull up and cover the face. There are some 10th/11th century helmets that have been preserved with nasal and cheek 'hooks' to allow for a face-veil of armour to be hung from, but the contemporary pictures do not portray it like that. Also it would severely limit movement of the head. That said, it would also protect the wearer from slashes to the throatNeither theory is conclusive, so use whichever theory you like or ignore both and use any other conjecture that springs to mind.
I guess that it will always be a point of conjecture.