I think the very first ones were tube, and they had aluminium ends joining it all together.
Later ones were pressed - indeed, maybe out of a single sheet folded over and riveted - there's a 'bead' along the outside of the short connector between the two boxes on the later version (Mark IV) which suggests it was a flat part folded over in two after pressing both halves of the boxes in.
Yes, the ends are pressed - three designs as far as I know - plain circle, and long and short offset tabs.
Nick