Using FastLane's online catalogue, searching your year and model I found the tensioner at the foloowing link.
This and all the tensioner items can be found under the cooling system link, many have pictures to help ID the part in question.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.