I may be way out to lunch on this, and someone will surely correct me if I am wrong----I believe that these tanks had a manual air pump on them, and could be pumped up with air pressure, to pressurize the fuel into the carburators on a 1/4 mile drag race. If your engine was "hopped up" and multi carbed, the stock mechanical fuel pumps of the day simply couldn't deliver enough fuel to the carburators for a top speed run. There was a "switch over valve" that allowed you to run off the gas tank for normal around town driving, but at the drag strip you ran off pressurized fuel from the front tank. You could also run a more potent blend of gas from the front tank, that was O.K. for a blast up the drag strip, but would cook your motor or your wallet if used for normal driving.