As a follow-up to more questions I received:

Yes, it works also on OM603 and OM606 engines. It works on N/A as well, but I suggest its application with turbo engines.

The spring is stainless spring steel, calculated for the Mercedes engines and fatigue tested.

With the spring (actually rebuilt valve) a turbo engine will get more fuel/power without modding the pump on a bench. (It works on a NA engine as well, but as I indicated, the most gains are felt on a turbo engine).