These are the ones you are looking for:

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ42970VVXEC&year=1982&make=MB&model=300-DT-001&category=All&part=Center+Cap

I don't have the size in mm or the part number.