Remove the back "cover" (it slides down and out) of the driver's seat --- screw under the back edge in the middle of the seat. On a w126, there are 2 screws; I think a w123 has ONE screw.

The arm rest has a bolt that runs thru it and attaches to the inside of the seat frame. With the back cover of the driver's seat off, you can use a socket and ratchet to remove the nut holding the arm rest on the seat.

It's either a 19mm or 22mm sized nut.

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daw_two
Germantown, TN

Links:
Sold last car --- 05/2012 1984 300D Light Ivory, Red interior
No longer selling Cluster Needles Paint
No longer selling New Old Stock (NOS) parts

Past:
3/2008 1986 300SDL "Coda"
04/2010 1965 190D(c) "Ben"
& many more