Bill,
On my tank, there is a lower portion towrd the rear on one side, only, but the taps do not occur that far back. Therefore, there is no tap in the lowered portion. Also, I still don't see how a tube on the opposite side of the tunnel increases the reserve, unless it is a really long tube. I think the blokes (if I may use term) at BSA who designed the tank sheet metal were not the same ones deciding where the taps should go. Do you have another explanation?
Richard