I couldn't find the post with the pictures of 126 seats in a 300CD, but here's a post from ICEFIRE from back in 12/03/06:

"Hey guys I thought I would share this little tidbit with you, It is relatively easy to to swap in W126 seats into a W123,

You need to unbolt the W123 front seats from their rails, this is done in some cases by removing 6 bolts, or in some cases it is necessary to remove 12 bolts. The frame rails need to be aligned into certain postions to remove the bolts. The will either have a torx + flathead head, or a philips, not both.
Bolt the frame rails onto the W126 seats. In some cases the seatbelt reciever will need to be changed. Some simple wireing will take care of the recline & headrest wireing on the 126 Seats, (2 DPDT switches per seat).

The front seats fit perfectly, the rear seat fit 98%. Of the three tabs that hold the top of the rear seatback in place, bend the center support toward the front of the car about .5 inch. you will need to drill the 2 new mounting holes left and right lower bolt brackets, but this will be completly clear when the seat is in place.

The rear seat bottom is about .5 inches to wide, but fits acceptably, you will need to drill 2 holes for the peg like things that secure the W126 rear seats.

All in all pretty easy job. Couple of hours probably.
So if those MB Tex 123 Seats are on their last legs and the local wrecking yard has some mint 126 seats, I say go for it, I would do it again if I owned a 123, this one was for a client..."

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Houston, Texas
1984 300CD 235K miles, Elvira, one owner
1987 300TD 180K miles
1974 TR6 78K miles Sarah Jane, one owner
OBK #27