The bar/driveshaft that connects the lateral hinges for the seat back recliner is interchangeable between the 123 and 126. I know this becuase the passengers seat in my 560SEL broke and was stuck in a semi-reclining position and only the right side would lift/lower. I took apart an old 123 driveshaft I had (which has a square profile) and it fit perfectly and now my 126 seat works great. Now, does this translate into the rails/mount points being the same as the 123, I'm afraid I dunno, but physically the seat should fit in the same space.

I say give 'er a shot