Dave - there are 2 on your car - photos below(I hope). Which did he do? Click on the photo for parts price.

Hopefully the URL below will bring up a photo of the front/rear disc.

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1EQ0PTV2S1JL0YQK8P&year=1989&make=MB&model=300-SE-001&category=K&part=Flex+Disc

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'91 300-SEL