I think I got it! The stitching style of the seat depends on the back seat. Wait hear me out...I am a valet driver so I get to drive e30's every once in a while. I have noticed that e30 cabrios always have the horizontal (M3 style) stitches, and other e30's always have the vertical (325is style) stitches. What do e30 cabrios and e30 M3's have in common? They only have 2 rear seats! So is it safe to assume that the horizontal (m3 style) stitching came on e30 cabrios and e30 m3's, and that the vertical style stitching (325is style) came on regular e30's? All e30 m3's and e30 cabrios only have 2 rear seats right? All regular e30's have 3 rear seats right?