Hi Alan.
From a study of a photo of No.6 on the dump sidings, I am very confident I can make out the detail of the outer face of the tank.
I can see a join between the outer plate and a flange on the inset middle, as on your left hand sketch.
The photo doesn't show the join on the inner tank face, but I would assume it's the same.
Also, I can't tell whether the join is formed by bending the edges of the middle plate, or with a separate strip of angle iron. Your call.
Steve.