Root wood does not have a straight grain; see how it swirls around in the photos. This makes it harder to split. Bog oak is no more like ordinary oak than tanned leather is like rawhide; it has been put under pressure and chemically transformed. It may be the thicker rim of the bowls is to help with splitting, as that is where they will start to split. The massive rims make it more likely they are helmets, as thick rims like that make pouring really messy. You wouldn't do that with a food bowl.

Were metal helmets also known in the area? If they were, it's pretty likely that these "helmet bowls" might have been ceremonial imitations with some sort of ritual or magical function, and if so then it'd make sense that we don't see any attachment points for a lining or suspension system.

Regarding chinstraps - how much evidence do we have in general for use of chinstraps prior to 13th century? None of Vendel period helmets have any signs of chinstraps, for one. I am no expert on Roman helms, but I do not remember seeing any indications of their use with Monteferrino style helmets, for example. ...

Oh, the Romans LOVED their chinstraps! Usually they were fixed at the center back, crossed under then chin, and then fastened to rings or studs on the cheekpieces. Greek helmets usually show a pair of holes somewhere for a chinstrap, though not always--we're not sure if the rest might have been sewn to the lining, or what. A full-face type like the Corinthian might not always need a chinstrap, but I suspect the simpler bowl shapes like these wooden ones would tend to have something.

Matthew

I don't think we have good enough photographs to say anything about whether the 'helmets' show any sort of attachment points. Perhaps we expect to see large holes right through the wood, when that may not be how it was done.