I'm inclined to agree that it's not a ventail, since it's never shown used as one, and it's displayed square in the middle of the chest on hauberks being worn and hauberks hanging on rods. It's definitely not leather trim around the neck line because it's depicted in the middle of the chest while worn.
I had to look at the pic again to spot the square on the one guy where it is being worn, I missed it earlier. I'm not sure what is going on with that guy.
The ones on the pole, I still believe are leather trimmed necks. A properly tailored haubergeon / hauberk isn't symmetrical front and back, and the back will have a straighter cut than the front of the neck, and look "open" from the front, similar to how it sits on the pole. The edging at the end of the sleeves and lower hem seems to support the leather used specifically for edging, and although I've only done so around the house so far, I've noticed less "snagging" with a leather trimmed edge by having fewer "loose" rings to catch easily and letting the mail "slide" more naturally when going on/off - this may be a modernism due to the way our modern indian produced mail has the lumpy rivets vs proper smoother historical mail, so I don't consider it as anything other than personal experience. I did take my inspiration on the leather trim from the Bayeaux Tapestry images a couple years back.
See these people, also from Bayeaux Tapestry (Battle of Normandy), who seem to be wearing mail with a leather collar or trim. Note that none of them have a square of leather on their chest. They also seem to have the same sleeve and hem leather trim, not likely to be an undergarment since it follows the flow of the mail and sticks out, which makes me suspect it is attached/laced together in some way.
King Harold and someone next to him (off to the right) not only seem to have leather trimmed sleeves, but a second sleeve of mail underneath, with it's own leather trim. He is also "missing" the square reinforcing bit in front, and if the pattern of mail in the picture can be taken as generally correct, it appears his coif is worn inside the haubergeon, and the haubergeon neck has leather trim.
None of them have a raised ventail either, all picture with open faces / norman conicals, which I think further shows it likely was not an opened ventail. If the King himself didn't have a ventail, I would be surprised if other soldiers did. I do wonder if Harold had a hauberk under a haubergeon for substantial double-layered protection, due to the prominence of his seemingly double-sleeves.
