Hi Simon,
Only one "C" shaped support seems to make sense... The steel plate thickness of the exterior metal plating is 11.5 mm for R1, R2, and Q Tank (.45 inch). If the internal longitudinal supports, and the internal bulkheads were the same thickness as the outer plating, then all would have to be formed at a steel pressing plant and be assembles/welded together on site.
The saddle tanks FTB2 and FTB4 were 6 mm thick (.23 inch) for the outer shell.
Regards,
Don_