1985 190D 2.2: where is the fuel prefilter?

I'm perhaps exposing my ignorance, but where is the fuel prefilter on my engine? The picture included with the filter kit doesn't help me, as I can't locate just where that refers to. I suppose it's possible that the prefilter was removed at some time.
Since I plan to replace the gunked-up filter in the tank, I'd like to have a good prefilter in place to catch whatever I stir up in the process.
- Richard

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