If you have the jumper in place you can see it. It is just a "u" shaped bare wire (just like the opening part of the padlock) pushed through the two holes in the back of the cluster near the "padlock" symbol.
Not sure which models had the larger tank capacity but the UK Haynes manuals mention an optional aux. tank with it`s own sender unit, for the wagon models, I think. Also there is a longer tank sender unit/in-tank pump unit listed for later turbo models which implies a bigger single tank.
No doubt someone with more info on model specs will tell us the details.
Don`t place too much faith in fuel gauge readings as they are not exactly precision devices at best, and wildly innacurate at worst!
Colin.