I have always wondered about that! I doubt that the stitching are different due to the material though. I have a 1988 325is with the vertical stitches. It is my understanding that 325is' always came fully loaded with every option including leather. By the way: I am getting my seats reupholstered very soon. Do you think it is possible to get them to reupholster it with the horizontal (M3 style) stitches rather than my current vertical (325is) style stitched. The m3 style horizontal stitched seats look much better in my opinion.