Julian
The brass tube is for the smokebox proper. It has a wrapper made from 22 swg steel shaped as you describe. I assume that the external view of the smokebox is an upside down 'U' to fit the smokebox front, with a normal cylindrical smokebox under the wrapper. It is not very obvious from the drawings that that is what is meant.,
Thanks Mick H but Kennions tube is copper in that size but I appreciate the link as I had forgotten about Kennions!
Malcolm