Arm rest locking mech. is behind the large push button on the arm rest. Button is held in by plastic clips moulded into the button. then you can get to the nuts inside. At least that's how the 126's I've worked on come apart.

Did you happen to look at the inside of the seat frame and see IF there is a large hole in the center of the bolt pattern?? If so just open a hole in the leather and bolt it up as is, use one screw for a lock pin. (??)

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1986 Euro 500SEC "RUF" 9:1CR, "Rose"
1985 Euro 500SEC Cabriolet AMG Widebody
1982 Euro 500SEL "Blue"
2001 Texas Heeler (Aussie/Queensland X) "Sulphie",
2012 Queensland Red Heeler "Squeak"
Best dogs I've ever had.