Here's something interesting
 

The stock steering wheel on my 1991 420 SEL and my 1991 560 SEC have the same rim thickness. However, the rim thickness on the stock steering wheel on my 1991 560 SEL is decidedly thicker and feels, therefore, a bit more contemporary. I haven't measured them to confirm this "feel" but my wife noticed the same thing. Was there an upgrade put into production somewhere? Again, all 3 are stock. If anything, I would have thought the coupe would sport the thicker steering wheel rim.
J.Chip