Just click on the Car icon, go to 211 and scroll down to your model which is 211.220
That will give you all the EPC for the car.
I've done it for you below.
Parts catalogs. Spare parts catalog Mercedes EPC Online. Mercedes. Car. Mercedes FG 211.220 (65J).
nemigaparts.com
The only link I can see for an idler pulley is below, but it has no Part# ??
Parts catalogs. Spare parts catalog Mercedes EPC Online. Mercedes. Car. V-BELT,BELT TIGHTENER,SHEAVE PULLEY.
nemigaparts.com
You may have to get that number from the main dealer.