Square Tank bolted to side on my 1980 300sd

On this 1980 300sd I have a square fuel tank that is bolted to the sides. Thers is no fuel sensor on the top of the tank. I can see all the way into the back and there is no sending unit. Where is the sending unit on these early 116 300sd's?

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Diesel Engine Specific Aid

Chassis Model Number

W116

Not sure you mean by square

The factory 116 fuel tank was not square. Shape is similar to 123 chassis tank. Sender on top. Maybe your tank is not factory....

I am of the thought that

I am of the thought that there are 2 tanks in the trunk. There is the normal drain on the bottom and this square tank has a hose going across the top as almost or probably an overflow. The fill itself is different than my other three tanks. I am trying to decide if I want to take it out to clean it or leave it and just use startron in it. I wonder how they would be connected?