So THAT's what the gauge on the top left is for! I too thought the trip odometer was standard equipment on MBs to ensure the driver can figure out how far they can go before the tank is dry.
I have Dennis Adler's "Early Speed and Opulent Luxury: The great Cars", and on page 30 they show a beautifully restored 1915 Oakland Model 37 Speedstar with a gauge on the gas tank in the back. It was a new convenience, designed to improve on the old "walk around the back and dip a stick in it" method to check the level. How many current MB owners would use the stick method today if the tank fill line didn't bend?
My 1980 300D (235k miles) has an erratic gauge and no reserve light function. My 1991 300TE 4matic has the above mentioned inaccuracies < 1/2 tank and a working reserve light. The reserve light is a very helpful tool. I don't know what I'll do if the trip odometer dies!