An excerpt from the 300 service manual:
"This vehicle is available with either standard gascharged
shock absorbers or Nivomat™ load-leveling
shock absorbers for the rear suspension. Depending
on options, the load-leveling shock absorbers may
either be standard or optional equipment.
On the exterior, the load-leveling shock absorbers (1)
are larger in diameter than the standard shock absorbers.
Each load-leveling shock absorber is a self-leveling,
self-contained vehicle leveling system and shock
absorber combined. It does not require any external
compressor, hoses, or height leveling sensors. When
cargo or passengers are added, the load-leveling
shock absorbers use normal ride motions during driving,
even on smooth roads, to raise the rear of the
vehicle automatically to the unloaded (or curb) height.
When the load is removed, the vehicle maintains this
rear ride height."