Google Search = Viton O-rings for mercedes diesels
Went down the list of sites and found:
http://www.threepoints.com/forum/f30/om602-diesel-oil-stem-o-rings-viton-o-rings-biodiesel-136/
Has a list of O-ring sizesm for a OM602. Someone else can look up the part numbers for a OM602 and OM603 Fuel Injection Pump O-ring and see if they are the same part numbers.