Historian J. R. Planche argued in 1847 that it was a "square pectoral" used for an extra layer of protection.
I'd be cautious of that, as the Victorian era is responsible for a plethora of armor destruction and inaccurate "knowledge".
Looking at the first pic you posted again, I enlarged it, and I'm wondering if he has a leather neck liner as in the ones that I posted, supplemented by the "varangian bra" that Thorsteinn posted. It would look essentially square from certain angles. Any other pics showing the square on the chest area? I'm quite curious now.