Type this,swap 123 seat into 126 , in to forum search and you will get 4 or 5 threads that describe this. I was interested in swapping 300SE seats into my 380SE.

Right now I'm working on swapping my passenger seat out for my Driver seat. Driver seat has seat memory and the passenger doesn't and the motors will need to be swapped as well.

The assumption that both the 126 driver and the passenger seats have the same frame is a correct one. the passenger side leather merely covers the armrest/mtg plate holes. Now IF the seat back from the passenger side will cover the drivers side holes in the leather at the armrest, then I'm home free.
My passenger side seat bottom sits about two inches higher than the mushed drivers seat bottom with my big rump in the seats. So this should be a good swap for me. They both need to be recovered, but that can wait awhile. I just bought a like new armrest off Ebay. now I'm looking for two seats to match it's condition.

__________________
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.