your 'mystery' tank is the expansion tank for the fuel tank. When you over fill the tank with cold fuel and it heats up, it will flow into this tank instead of onto the ground. Not all models have them.

The "reserve" is how much fuel is left when the low fuel light comes on.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!