Just in case you want a comparison to FastLane's price, my local MB dealer wanted $21.75 for the W124 headrest adjuster (P/N 124-970-01-26). My MBCA member discount knocks a couple of bucks off that.
Not sure why the part number is slightly different from that posted earlier in this thread, but this is what is on my receipt.

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DavidB29
1992 300E with ASR
35 years of Diesels until now!