Late Roman helmets with one-piece bowls

[Robert Vermaat]

I've been researching the Sutton Hoo helmet, which appears to derive from the fourth and fifth century helmets of the Constantinian workshops. Curiously, the bowl of the Sutton Hoo helmet appears to have been raised from a single sheet of iron, whereas the Roman examples are pretty much universally of bipartate or quadripartite construction (James 1986). Helmut Nickel, however, states that the one-piece bowl technique was not lost until around 500 AD—incidentally, the earliest suggested date for the Sutton Hoo helmet.

If the skill needed to make one-piece helmet bowls was around until 500 AD, are there examples of late Roman helmets made in such a way?

I have some doubt about the Sutton Hoo helemet 'deriving' from Roman helmets in a direct line. Ridge helmets had disappeared long before the construction of the Sutton Hoo helmet was made (I doubt that the SH helemt was that old btw), being replaced by spangenhelmets etc perhaps a century earlier?
The SH helmet may be more akin to Persian ridge helmets, who originally seem to have inspired the Roman ridge helmets in the first place, and which seems to have continued long after the Romans stopped producing them.