Eric,
I don't think it will work. I believe the attachment points are different. Also, the shape of the floor under the seats themselves is different, so the cushions won't sit right. Since you have a W123, you know that there is a locking bar that goes across the width of the seat. This is missing on the W124.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member