The insert with the 3 holes is actual an edge Filter.
The Older VW Rabbit and Volvo Diesel Nozzles that physically look the same and are otherwise interchangeable don't have the Edge Filter. In fact most other makes of Diesel Injectors don't have the Edge Filter inserted into the Injector.

What is it there for? I don't know if this explanation is true or not. I was told that as the Fuel Injection Lines erode inside over time it is possible for small pieces inside to break off and make their way to the Injector and the Edge Filter is there to keep them from getting any Further.
However, as I said the bulk of the Injectors out there don't have that Filter so it I doubt if the problem happens frequently or all Injectors would have the Edge Filter.

Or the Explanation I was given is entirely wrong.

The first set of Injectors I rebuilt and put into the Mercedes used VW Rabbit Injector Bodies that did not have the Edge Filter and there was no difference caused by them.

I have seen some of the Older Mercedes Injectors that don't have the Filter either. All of them were 4 Cylinders.

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84 300D, 82 Volvo 244Gl Diesel