slip differential

Hello to all, I have to refurbish the differential of my 1996 BMW Z3 1.9. I have never put my hand to this differential and at first sight I find it different from the standard LSD ones where on the cabinet there is the letter "S" while in that of the Z3 there is the letter "R" someone knows...

FOR SALE - 2.2i - 2001. 95K - With Service History. Recent Service with oil/filters/plugs Uprated Disc Brakes and Pads all round (1 month ago) 17 Inch Z4 Alloys with recent tyres Roof in superb condition with no leaks Rare LSD Option M-Suspension Option Side Airbags Removable CD Stereo with USB...