The spacer is on the crankshaft behind the balancing disc. The seal lip actually rides on the spacer. Typically, the old seal will have worn a groove in the spacer, so a new one will go a long way toward ensuring that the new seal does not leak. Sometimes, the wear is offset from the center of the spacer enough that the spacer can be reversed and reused. Other times, the wear is too close to the center and you need a new spacer. Apparently, not all seals are created equally when it comes to depth.

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2AT0J1QHK2AX17B7Y0&year=1983&make=MB&model=300-SD-002&category=A&part=Crankshaft+Seal+Spacer