I have an '83 300SD and, to my knowledge, there isn't any type of reserve that requires flipping a switch to activate. Therefore, the 12 L reserve that your shop manual mentions is most likely just factored in to the overall capacity of the tank. I feel fairly certain that the low-fuel indicator light coming on is what they mean by stating that it has a reserve. The 12 Liters for the reserve sounds about right since my low-fuel light comes on when the needle gets slightly below the 1/4 tank mark. At 77 Liters, 1/4 of a tank would be 19.25 liters so a 12 liter reserve would make sense. If I were at home I might be able to confirm since my manuals are all there. One other thing that leads me to believe your suspicions are correct is that if the 12 liters were in addition to the 77, that would mean an 89 liter tank or 23.51 gallons. That would cost me over $30 to fill up and I don't recall ever spending more than $23 or $24 to fill up. By the way, your fuel mileage seems to be pretty good.