Michael,

The armrest on my 92 400E only has piping on left and right sides, at the back of the armrest, the upside and downside leather jointed by a hidden flat buckle on both edges, I accidentally found it.

Not sure about other W124s.

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99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.