The twin top tubes, "Lats" as they are known in mixte speak are 16mm (⅝ inch) diameter. They are either seat stays or chain stays...I'm not sure. Probably seat stays. But I do know they are 0.8mm straight gauge 4130 cro-mo. To my understanding they will be spliced with round s-bend seat stays to achieve the proper length. These should do wonders in the torsional stiffness department.
Also present in the above photograph are the newly machined (today) head tube reinforcement collars, a 73mm Birmingham Small Arms "British Standard" bottom bracket and the seat tube/twin lats lug that needs some minor modification to accommodate the larger than usual lats.
It is hard to tell but that's the head tube over there on the left. The first piece is in the jig!