I was wondering if that were it, but the shape is pretty definite – the things are straight sided or even slightly belled out at the top (hence my shako comment above). Presuming the draughtsman drew what he saw and was good enough to set it down accurately (his other figures look pretty good to me) that’s what they looked like.
I wonder if this is more the sort of thing – as I think OB suggested – more of a great helm? The odd(er) thing is that they appear to be wearing them on rather than over their heads – too small? Fashion statement? Bit of a laugh? Something else altogether?
Great series of posts on OB’s blog, whatever the truth of this illustration.