Interesting point. In theory when you fill up, the reserve gets filled first, and should mix with the old in there. Then after you get a few gallons in there, it should start spilling over into the main center side, and flow to the right side as well.
Maybe if the fuel in your reserve got really really old, like approaching gummy goo, it would stay at the bottom, and the fresh clean fuel would not mix, but flow over the top into the main side. Just a guess.
It brings another thought, that we should make it a habit to use the reserve once in awhile, and use it up before putting the car into hibernation every winter.
Earlier in this thread I had mentioned the possibility of different tanks.
Here is why.
This is from the 1938 brochure that introduces the 3-1/2 Litre.
Anybody know what this is all about? Is that a '37 tank? Its certainly nothing like my tank.