That's hard to say being that I don't ride back there. The seats in the 124 are not as tall as the 123's. There is less spring travel. The springs in my 123's rear seats are fairly firm, whereas the 124's, I would call stiff. It seem they used more foam in the 124's and the 123 has the horsehair pad and metal springs.

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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