The expansion tank is a major component of the engine cooling system.
It is attached by a medium hose to the radiator, and 2 to 3 smaller lines to take up the coolant from the system as it heats up and expands. When the engine cools down, the coolant flows back into the radiator.
Early versions of the R129 SL used a metal cap on the tank that is spring-loaded to release pressure if the temperature gets too high. Later version use a plastic threaded-on cap like this one with an O-ring seal to do the same thing.
The tank sits on the right-front passenger-side fender and is mounted with plastic to rubber grommets and two 10mm nuts. Inside each of the hose attachment points is a metal ring. This keeps the plastic from deforming and cracking when you tighten down the hose clamps.
There is an electronic sensor that monitors the fluid level in the expansion tank. If the level is too low, a light on the dash comes on. The sensor fits into a small tube cast in the plastic bottom of the tank behind the main hose connection and locks with a twist.
When the system is cool, you can check the coolant level by removing the cap and looking where the level is relative to the vertical rib cast just below the fill neck. If it is low, you can add some coolant mix or distilled water, but if you find you are doing this often, it means there is a leak somewhere in the system and you have to fix that.
The coolant is a mixture of distilled water and anti-freeze. The optimum ratio is 50% of each. 100% anti-freeze does not improve the low or high temperatures function, and is an expensive way to fill the 2.5 gallon (9.5 liter) system. Straight water will boil at 212°F (100°C) and freeze below 32°F (0°C). With the 50/50 mix, your system will be able to operate well outside the thermal range of just water.