Did you find a w115, rare option here. It is the same set up as the w123 and was used in the w116 on the 6.9. The struts replace the shocks and are dampened by gas charged spheres. The springs are still used. The system works great in cars like the TD wagon and taxi sedans where there is excessive weight in the rear of the car. The SLS system keeps rear ride height constant regardless of the load.
Now the 6.9 used the system on the front and rear while the 115 / 123 was just the rear. The 6.9 used different rear control arms. Any of the set-ups required a boss on the engine head or block for the suspension fluid pump which ran off the timing chain...
I could see the rear SLS work in a W116 300SD using a 617.951 from a 300TD wagon.

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Over 21 years I owned several:
w108
w110
w111
w115
w116
w123
w124
w126
w212