Whichever one goes bad. Yeah I checked my ride out and the part number stamped on it is 1202000470 like you stated. I have a new NSK supercharger bearing on its way too. Thought I would do them both.

this correct?

http://www.ebay.com/itm/New-Mercedes-Benz-W140-S600-R129-SL600-Belt-Pulley-1202000470-/281424174166?hash=item4186301c56&vxp=mtr