The rear axle hydropneumatic (nitrogen) compensating spring is a distinctive W111/W112 feature, and knowledgeable buyers want the original retained rather than swapped for a cheap aftermarket coil kit. The originals are no longer available new but are rebuildable, which commenters consider well worth doing.
Rear hydropneumatic compensating spring
Mercedes-Benz W111 Coupe / Cabriolet · mechanical